Optimization of Peripheral Blood Stem Cell Mobilization

Peripheral blood stem cells (PBSC) are increasingly utilized in lieu of marrow for hematopoietic support due to the ease of collection and the rapid kinetics of recovery relative to bone marrow (BM). Neutrophil and platelet recovery times after PBSC transplantation average less than 8‐12 days after infusion in contrast to the usual two to four weeks experienced after BM transplantation. This has simplified autologous transplantation and made it safer because patients require fewer days of antibiotic and blood component support and are discharged earlier from the hospital. The administration of hematopoietic growth factors during recovery from high‐dose chemotherapy increases the number of circulating hematopoietic progenitor cells to levels as much as 1,000‐fold greater than levels normally found in blood and 10‐50 times greater than with chemotherapy alone. More recently, it has been shown that adequate numbers of PBSC can be collected using growth factors alone without prior chemotherapy. Although not yet universally accepted, the CD34+ cell content of PBSC appears to be the single most powerful predictor of recovery kinetics in patients receiving myeloablative therapy and PBSC infusion. Infusion of >5 × 106 CD34+ cells/kg is associated with a rapid engraftment of neutrophils and platelets, although successful engraftment has also been reported with the infusion of 2.5‐5 × 106 CD34+ cells/kg. By measuring the CD34 or colony forming units‐granulocyte‐macrophage (CFU‐GM) content of PBSC collections, mobilization chemotherapy and cytokine regimens, age, marrow disease, prior radiation and prior chemotherapy treatment have been found to be important factors influencing the numbers of stem cells collected. The current challenge for clinical investigators is to improve methods of identifying patients who will fail to mobilize sufficient numbers of PBSC prior to collection and to utilize new strategies for stem cell mobilization. The relative ease of collection and the rapid engraftment after myeloablative therapy suggest that PBSC will likely supplant marrow for both allogeneic and autologous transplantation in the next five years.