Successful mobilization of peripheral blood stem cells using recombinant human stem cell factor in heavily pretreated patients who have failed a previous attempt with a granulocyte colony-stimulating factor-based regimen

The current status in hematopoietic stem cell mobilization

Hemotopoietic stem cell mobilization with cytokines alone, has still been widely accepted as the initial attempt for stem cell mobilization. Chemotherapy based mobilization can be preferred as first choice in high risk patients or for remobilization. But mobilization failure still remains to be a problem in one third of patients. Salvage mobilization strategies have been composed to give one more chance to 'poor mobilizers'. Synergistic effect of a reversible inhibitor of CXCR4, plerixafor, with G-CSF has opened a new era for these patients. Preemptive approach in predicted poor mobilizers, immediate salvage approach for patients with suboptimal mobilization or remobilization approach of plerixafor in failed mobilizers have all been demonstrated convincing results in various studies. Alternative CXCR4 inhibitors, VLA4 inhibitors, bortezomib, parathormone have also been emerged as novel agents for mobilization failure.

Advances in stem cell mobilization

The use of mobilized peripheral blood stem cells (PBSCs) has largely replaced the use of bone marrow as a source of stem cells for both allogeneic and autologous stem cell transplantation. G-CSF with or without chemotherapy is the most commonly used regimen for stem cell mobilization. Some donors or patients, especially the heavily pretreated patients, fail to mobilize the targeted number of stem cells with this regimen. A better understanding of the mechanisms involved in hematopoietic stem cell (HSC) trafficking could lead to the development of newer mobilizing agents and therapeutic approaches. This review will cover the current methods for stem cell mobilization and recent developments in the understanding of the biology of stem cells and the bone marrow microenvironment.

Mobilization and homing of hematopoietic stem cells

Hematopoietic stem cells (HSC) are a population of precursor cells that posses the capacity for self-renewal and multilineage differentiation. In the bone marrow (BM), HSCs warrant blood cell homeostasis, but at the same time a stable pool of functional cells must be constantly maintained. For this, HSCs constitute a model in which subpopulations of quiescent and active adult stem cells co-exist in the same tissue, in specific microenvironment called stem-cell "niches." These microenvironments keep the stem cells at quiescent (osteoblastic niche) for its self-renewal and activate the stem cells (vascular niche) for proliferation and/or injury repair, maintaining a dynamic balance between self-renewal and differentiation. HSC reside in the bone marrow but can be forces into the blood, a process termed mobilization used clinically to harvest large number of cells for transplantation. At the same time, homing to the BM is necessary to optimize cell engraftment. Here, we summarize current understanding of HSC niche characteristics, and the physiological and pathological mechanisms that guide HSC mobilization both within the BM and to distant niches in the periphery. Mobilization and Homing are mirror process depending on an interplay between chemokines, chemokine receptors, intracellular signaling, adhesion moleculas and proteases. The interaction between SDF-1/CXCL12 and its receptor CXCR4 is critical to retain HSCs within the bone marrow. Current mobilization strategies used in clinic, mainly G-CSF cytokine, are well tolerated but often produce suboptimal number of collected HSCs. Novel agents (AMD3100, stem cell factor, GROßT.) are being developed to enhance the mobilization to modify the signaling into the niche and boost the stem cell harvest, increasing the number of HSCs available for the transplant.

Mobilization of hematopoietic stem cells into the peripheral blood

Hematopoietic stem cells can be mobilized out of the bone marrow into the blood for the reconstitution of hematopoiesis following high-dose therapy. Methods to improve mobilization efficiency and yields are rapidly emerging. Traditional methods include chemotherapy with or without myeloid growth factors. Plerixafor, a novel agent that disrupts the CXCR4-CXCL12 bond, the primary hematopoietic stem cell anchor in the bone marrow, has recently been US FDA-approved for mobilizing hematopoietic stem cells in patients with non-Hodgkin lymphoma and multiple myeloma. Plerixafor and myeloid growth factors as single agents appear safe to use in family or volunteer hematopoietic stem cells donors. Plerixafor mobilizes leukemic stem cells and is not approved for use in patients with acute leukemia. Patients failing to mobilize adequate hematopoietic stem cells with myeloid growth factors can often be successfully mobilized with chemotherapy plus myeloid growth factors or with plerixafor and granulocyte colony-stimulating factor.

Current status of stem cell mobilization

New advances in effective mobilization of peripheral blood stem cells have permitted a greater proportion of patients to benefit from autologous stem cell transplantation. In this review, the relative merits of peripheral blood and mobilized bone marrow are discussed. All available agents are reviewed. A critical assessment of the appropriate dosing and frequency of available growth factors is undertaken, and the most commonly used chemotherapy plus growth factor combinations are covered. Specific recommendations for patients who are poor mobilizers are dealt with including the role of plerixafor.

Ancestim (r-metHuSCF) plus filgrastim and/or chemotherapy for mobilization of blood progenitors in 513 poorly mobilizing cancer patients: the French compassionate experience

Ancestim (r-MetHuSCF) is available in France for compassionate use in patients who are candidates for high-dose chemotherapy and autologous transplantation, and who failed in previous attempts at mobilization and collection. We report here data from 513 adult patients who benefited from this program, between January 1998 and July 2007. Given with systematic premedication, ancestim was generally well tolerated, although severe but not life-threatening adverse events were reported in 12 individuals. Overall, a graft was obtained or completed for 235 patients (46%). The median number of collected CD34+ cells was 3.00 × 10(6)/kg (range: 0.03-39.50). The target threshold of 2 × 10(6) CD34+ cells/kg was reached in 161 patients (31%). Factors associated with collection were diagnosis of myeloma, no previous autologous transplant, no more than one previous failed attempt and a mobilization regimen including cytotoxic agents. A total of 207 patients (40%) proceeded to high-dose chemotherapy and autologous transplantation. The median time to reach 0.5 × 10(9)/L neutrophils and 20 × 10(9)/L platelets was 12 (6-40) and 13 (0-31) days, respectively. We conclude that a combination of ancestim with filgrastim successfully mobilized CD34+ cells in peripheral blood, and allowed adequate collection in preparation for autologous transplantation in approximately one-third of poorly mobilizing patients.

Management of poor peripheral blood stem cell mobilization: incidence, predictive factors, alternative strategies and outcome. A retrospective analysis on 2177 patients from three major Italian institutions

CD34+ peripheral blood hematopoietic stem cells (HSC) are usually collected following mobilization therapy accomplished by using growth factors (GF) such as rHuG-CSF or rHuGM-CSF with or without chemotherapy. A target dose of yielded CD34+ is usually prescribed by the attending physician depending on different protocols, which may include single or double transplantation. HSC collection usually is performed when at least 20 CD34+ HSC/microL are detected by means of flow cytometry. A cumulative dose of at least 2 x 10(6)/Kg/bw CD34+ HSC has been considered as the threshold to allow a prompt and persistent hematopoietic recovery. Unfortunately, this goal is not achieved by the totality of patients undergoing mobilization regimen. In fact, 5-46% of patients who underwent mobilization therapy fail HSC collection due to very low peripheral blood HSC CD34+ count. Patients' characteristics, including age, sex, stage of the underlying disease (complete or partial remission), diagnosis, previously administered radio/chemotherapy regimens, time-lapse from last chemotherapy before mobilization and mobilization schedule (including dose of GF) were considered as possibly predictive of poor or failed mobilization. We performed a retrospective analysis in 2177 patients from three large Italian academic institutions to assess the incidence of poor mobilizers within our patients' series. Therefore, a patient who fails a first mobilization (and when an HLA-compatible related on unrelated donor is not available) could undergo a second attempt either with different mobilization schedule or by using different GF, such as stem cell factor, growth hormone (GH), or more recently newly introduced drugs such as AMD3100, alone or in combination with rHuG- or -rHuGM-CSF. Thus, we investigated the fate of those who failed a first mobilization and subsequently underwent a second attempt or alternative therapeutic approaches.

The use of experimental murine models to assess novel agents of hematopoietic stem and progenitor cell mobilization

The recent explosion in the understanding of the cellular and molecular mechanisms underlying hematopoietic stem and progenitor cell (HSPC) mobilization has facilitated development of novel therapeutic agents, targeted at improving mobilization kinetics as well as HSPC yield. With the development of new agents comes the challenge of choosing efficient and relevant preclinical studies for the testing of the HSPC mobilization efficacy of these agents. This article reviews the use of the mouse as a convenient small animal model of HSPC mobilization and transplantation, and outlines the range of murine assays that can be applied to assess novel HSPC mobilizing agents. Techniques to demonstrate murine HSPC mobilization are discussed, as well as the role of murine assays to confirm human HSPC mobilization, and techniques to investigate the biologic phenotype of HSPC mobilized by these novel agents. Technical aspects regarding mobilization regimens and control arms, and choice of experimental animals are also discussed.

Hematopoietic growth factors for hematopoietic stem cell mobilization and expansion

During inflammation and cytopenia, increased levels of hematopoietic growth factors (HPGFs) induce mobilization and proliferation of hematopoietic stem cells and hematopoietic progenitor cells (HPCs), resulting in spatial and quantitative in vivo expansion of the hematopoietic tissue. Exogenous administration of recombinant HPGFs, particularly granulocyte colony-stimulating factor (G-CSF), is routine for mobilization of stem cells, followed by collection and transplantation of autologous or allogeneic stem cells. In this review, we summarize experience using different HPGFs and HPGF combinations for stem cell mobilization, such as G-CSF, granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), stem cell factor (SCF), and others. Preclinical and clinical studies of so-called early- and late-acting HPGFs for ex vivo expansion of HPCs are discussed, also with respect to the unresolved question whether expansion of repopulating stem cells can be achieved in vitro.

Cytokines and hematopoietic stem cell mobilization

Hematopoietic stem cell transplantation (HSCT) has become the standard of care for the treatment of many hematologic malignancies, chemotherapy sensitive relapsed acute leukemias or lymphomas, multiple myeloma; and for some non-malignant diseases such as aplastic anemia and immunodeficient states. The hematopoietic stem cell (HSC) resides in the bone marrow (BM). A number of chemokines and cytokines have been shown in vivo and in clinical trials to enhance trafficking of HSC into the peripheral blood. This process, termed stem cell mobilization, results in the collection of HSC via apheresis for both autologous and allogeneic transplantation. Enhanced understanding of HSC biology, processes involved in HSC microenvironmental interactions and the critical ligands, receptors and cellular proteases involved in HSC homing and mobilization, with an emphasis on G-CSF induced HSC mobilization, form the basis of this review. We will describe the key features and dynamic processes involved in HSC mobilization and focus on the key ligand-receptor pairs including CXCR4/SDF1, VLA4/VCAM1, CD62L/PSGL, CD44/HA, and Kit/KL. In addition we will describe food and drug administration (FDA) approved and agents currently in clinical development for enhancing HSC mobilization and transplantation outcomes.