A Standardized Cell Dose to Optimize Unrelated Donor Peripheral Blood Stem Cell (PBSC) Collections: A Prospective Study between the Sarah Cannon Transplant and Cellular Therapy Network (SCTCTN) and the National Marrow Donor Program (NMDP)

Background Allogeneic hematopoietic cell transplantation (HCT) depends on the capacity of donor hematopoietic stem cells to reconstitute recipient hematopoiesis after pre-HCT conditioning. Since the optimal CD34+ cell dose is unknown, transplant centers (TCs) request a wide range of cell doses, posing significant logistical challenges to unrelated donor PBSC collection and need for additional mobilization and unnecessary leukapheresis for high cell dose targets with risk of added toxicity to donors. Dose standardization may help determine a minimum acceptable PBSC cell dose to ensure engraftment and ensure donor safety without compromising HCT outcomes. Methods We prospectively studied unrelated donor HCT recipients at five SCTCTN programs from February 1, 2023 to July 31, 2023. Eligible recipients, who consented to participate in the CIBMTR database, included PBSC recipients of all age groups, conditioning, and types of GVHD prophylaxis. All diagnoses except for myelofibrosis were considered. A standard HSC collection of 5 × 106 CD34 cells/kg was requested for all patients. Donor collection characteristics were evaluated as well as the TCs adherence to the standard cell dose. Recipient neutrophil (ANC) and platelet (PLT) engraftment were measured. Results Median recipient age was 63 years (range, 21-77) and 58% were male. HCT indication was AML (50%), MDS/MPN (28%), ALL (13%), and other (9%). The median cell dose collected was 6.79 × 106 CD34+ cells/kg (range 2.44-35.61). Adherence to the standard dose request was 100%. Fifteen donors did not achieve the requested cell dose request (median 3.77 × 106 CD34+ cells/kg [range 2.44-4.98]), but only 8 were <4 × 106 CD34 cells/kg. Among all donors, 10% required a second collection day to achieve target cell dose. The median time to ANC and platelet recovery were 16 (range,0-27) and 21 (range, 0-167) days, respectively. Among evaluable patients, non-engraftment of neutrophils and platelets by day+30 was 1% and 10%, respectively. Conclusion In this ongoing prospective study of real-world unrelated donor HCT recipients, implementing a standardized PBSC dose across multiple sites was feasible and demonstrated acceptable engraftment. Overall, 90% of donor products were delivered to the TCs after just one day of collection. Future analyses will be presented including the incidence of primary and secondary graft failure, acute and chronic GVHD, transplant-related mortality, relapse, and survival. Standardizing cell dose requests may potentially improve donor safety, without risk to engraftment in the recipient.