Plerixafor As a Salvage Regimen to Mobilize Allogeneic Stem Cells in Healthy Volunteers (NCT01954914)

Introduction: Insufficient yield of Peripheral Blood Progenitor Cells (PBPC) from healthy donors can result in higher mortality after allogeneic transplantation. Plerixafor (Mozobil^®) is approved for the mobilization of PBPC in combination with G-CSF for collection and subsequent autologous transplantation in patients with non-Hodgkin lymphoma and multiple myeloma. Clinical data on G-CSF and Plerixafor in healthy donors are still limited. In a prospective single arm phase 2 trial we investigated the safety and efficacy of a single dose of Plerixafor as a salvage regimen in poor mobilizing allogeneic PBPC donors (NCT01954914).

Methods: We enrolled healthy unrelated PBPC donors who had received G-CSF (Lenograstim) at a targed dose of 10 µg/kg bodyweight (bw) for five days but donated less than 2x 10⁶ CD34+ cells/kg recipient bw during the 1^st apheresis on day 5 of the mobilization regimen. Plerixafor was injected subcutaneously at a dose of 240µg/kg once on day 5 in the evening before the second leukapheresis. The target for the processed blood volume during each apheresis procedure was 3x donor blood volume +/- 25%. Mobilization success was defined by a donation of more than 4.5x 10⁶CD34+ cells/kg recipient bw. Primary endpoint was the number of mobilization successes. The null hypothesis (success rate of less than 50%) was tested with an exact single sample binomial test at a one-sided significance level of 5%.

Results: Between 1/2014 and 12/2015 37 allogeneic unrelated PBPC donors (54% women, 46% men, median age 34 years) were enrolled into the study. The median donor bw was 69 kg (IQR, 61 kg to 76 kg) and the median recipient bw was 85 kg (IQR, 74 kg to 102 kg). Application of Plerixafor was well tolerated in most donors with only moderate side effects such as nausea, diarrhea and occasional vomiting. The median CD34⁺ count in peripheral blood was 15.4/µl (IQR, 12.0-18.3) on day 5 after G-CSF alone and 44.0/µl (IQR, 38.0-60.5) on day 6 after G-CSF plus Plerixafor. The collected total nucleated cell count was 187 x 10⁶ (IQR, 159.9-223.4) cells per ml volume of the apheresis product on day 5 and 289 x 10⁶ (IQR, 248.0-348.0) cells per ml volume of the apheresis product on day 6. The yield of CD34+ cells in the apheresis products was 1.05 x 10⁸ (range, 0.21-1.80) on day 5 and 2.8 x10⁸(range, 0.93-5.26) on day 6. The percentage of CD3+ cells per TNC was 31% (IQR 27%-40%) and 31% (IQR, 23% - 35%) on days 5 and 6, respectively.

In total, a median number of 5.16 x 10⁶ CD34+ cells (IQR 3.06-6.10) per kg recipient weight were collected during both aphereses. Twenty-one out of 37 donors reached the target cell count (56.8%, 95% confidence interval (CI) 39.5% - 72.9%) by donating a CD34+ cell dose of >4.5x 10⁶ CD34+ cells per kg recipient bw. Nine donors had donated ≤0.8 10⁶ CD34+ cells per kg recipient bw during their first leukaphersis after stimulation with G-CSF alone. Notably, only one out of these nine very poor mobilizing donors failed to donate at least 2.0 x 10⁶CD 34+ cells per kg recipient bw after salvage mobilization with Plerixafor. Nevertheless, the null hypothesis (H0: rate of mobilization success <50%) could not be rejected at the 5% level (p= 0.2557).

Conclusions: In this trial in poor mobilizing healthy unrelated donors the rate of mobilization success (donation of >4.5x 10⁶ CD34+ cells per kg recipient bw) was 56.8% (95%-CI 39.5% - 72.9%) after a single dose of Plerixafor on day 5 of G-CSF administration. After injection of Plerixafor the CD34⁺count in the peripheral blood and the yield of CD34+ cells in the leukapheresis product increased by more than factor 2. The application of G-CSF and Plerixafor was well tolerated by most of the donors. Like for autologous donors, the administration of Plerixafor appears promising to optimize PBPC mobilization in allogeneic poor mobilizing donors.