Single Autologous Stem Cell Transplant Using Busulfan and Cyclophosphamide as Conditioning Regimen in Patients with Multiple Myeloma

Autologous peripheral blood stem cell transplantation (ASCT) has been shown to improve survival in patients with multiple myeloma (MM). High-dose melphalan is considered the current standard of care among the preparative regimens used in ASCT for MM patients. We report the results of ASCT in 79 consecutive MM patients using a conditioning regimen with busulfan and cyclophosphamide (Bu/Cy), given as single ASCT and without maintenance therapy. Peripheral blood stem cells were mobilized with cyclophosphamide 5,000 mg/m² IV + etoposide 1,000 mg/m² IV, followed by granulocyte colony stimulating factor (G-CSF) 5 mg/Kg/day until the end of stem cell collection. A median of 41.1 × 10⁶ CD34⁺ cells/Kg (range, 2.1–139.7 × 10⁶) of ideal body weight were mobilized. The conditioning regimen consisted of busulfan 1 mg/Kg PO or 0.8 mg/Kg IV every 6 hours x 16 doses (days -7 to -3), and cyclophosphamide 60 mg/Kg/day IV for 2 days (days -3 to -2). Patients achieved neutrophil engraftment (absolute neutrophil count >500/μL) at a median of +13 days (range, +6 to +21 days), and platelet engraftment (platelets >20,000/μL unsupported by transfusion) at a median of 14 days (range, +11 to +24). Using a transfusion threshold of hemoglobin <8.0 g/dL and platelets <10,000/μL, patients required a median of 2 units of RBC transfusions (range, 0–8), and 1 platelet transfusion (range, 0–15) until hematologic engraftment. Forty-eight and 20 patients reached PR and CR, respectively, for an overall RR of 86%. At a median followup of 41 months (range 2–132 months), the estimated median overall survival (OS) and progression-free survival (PFS) were 45 months [95% confidence interval (CI) = 38–92] and 20 months (95% CI = 15–25), respectively. Veno-occlusive disease developed in 4 pts, and it was lethal in 1 of them. The Bu/Cy regimen was overall well tolerated, and transplant-related mortality was 4%. No statistically significant difference in terms of OS and EFS were observed between the group of patients receiving oral (n=13) vs IV busulfan (n=66). OS was not statistically different between the group receiving ASCT in first remission (n=62) and the group receiving ASCT as salvage therapy, i.e., upon MM progression (n=17), either calculating OS from the day of ASCT or from the day of MM diagnosis. We conclude that our reported clinical outcomes of the Bu/Cy regimen are equivalent to those obtained in historical controls with high-dose melphalan followed by a single ASCT. Thus, given the equivalent effectiveness but greater complexity of administration of the Bu/Cy regimen compared with that of single agent melphalan, we believe the latter should remain the agent of choice for ASCT in MM.