In fit patients with newly diagnosed myeloma, high-dose chemotherapy (HDCT) followed by autologous stem cell transplantation (ASCT) is considered standard of care. For mobilization of CD34+ cells for ASCT, combined cytotoxic chemotherapy and G-CSF is commonly used. However, the importance of cytostatic chemotherapy for reliable mobilization remains unclear.

This prospective randomized phase II non-inferiority trial compared G-GSF only (G) compared to standard chemotherapy/G-CSF (CG) for CD34+ mobilization. The primary endpoint was a less than 15% difference in successful stem cell collection (≥5·0×106 CD34+ cells/kg b.w. in a single day collection procedure without additional stimulation with plerixafor) with the G regimen.

136 patients were 1:1 randomized. 94 (69%) of all patients had successful stem cell mobilization with total collected CD34+ cells >5·0×106 CD34+ cells/kg in a single day apheresis procedure without additional plerixafor, 53 (78%) patients in the CG arm and 41 (60%) in the G arm (p=0·04). With an 18% difference in favor of the CG therapy, the non-inferiority margin was not maintained (95% CI 1%, 34%, p=0·04). The median total CD34+ yield was 9·99×106/kg b.w. in CG patients and 7·42×106/kg b.w. in patients with G-CSF alone (p<0·001). There were no differences in adverse events, hematologic engraftment, quality of life, or pain perception between the groups, but more patients in the G arm needed additional plerixafor with 18 (26%) patients versus 9 (13%) patients in the CG arm, respectively (p=0·06). Ultimately, 130 (96%) patients proceeded to HDCT with ASCT. The median time until neutrophil engraftment (neutrophil count >0·5×109/l) after ASCT was 11 (median IQR 10-15) days in CG patients compared to 12 (median IQR 11-17) days in G patients (p=0·31), and the median time until platelet engraftment (platelet count ≥20×109/l) was 12 days in both groups (p=0.17).

Our data indicate that G-CSF only is inferior to chemotherapy with G-CSF for peripheral CD34+ stem cell mobilization.