There has been a steady rise in the use of peripheral blood stem cells (PBSC) as a source of hematopoietic stem cells. In general, less than 0.05 % of white blood cells (WBC) are CD34+ cells. Chemotherapy results in a 5- to 15-fold increase of PBSC. Chemotherapy in combination with growth factors increases CD34+ cells up to 6 % of WBC. In the allogeneic setting, granulocyte-colony stimulating factor (G-CSF) is used alone for mobilization of PBSC. Several factors affect PBSC mobilization; including age, gender, type of growth factor, dose of the growth factor, and in the autologous setting, the patient's diagnosis, chemotherapy regimen and number of previous chemotherapy cycles or radiation. Muscle and bone pain are common side effects in allogeneic stem cell mobilization, but are usually tolerated with the use of analgesics. Spleen enlargement followed by rupture is a serious complication in allogeneic donors (Arslan and Moog, 2007).
While G-CSF is the standard mobilizing agent for PBSC donors, it is associated with some significant side effects and requires a multi-day dosing regimen. The other cytokine approved for stem cell mobilization, granulocyte-macrophage colony stimulating factor (GM-CSF), alters graft composition and may reduce the development of graft-versus-host disease (GVHD), but a significant minority of donors fails to provide sufficient CD34+ cells with GM-CSF and some experience unacceptable toxicity. Plerixafor (AMD3100) is a new mobilizing agent that may have several advantages over G-CSF for donor mobilization. As it is a direct antagonist of the interaction between the chemokine stromal-derived factor-1 and its receptor, CXC chemokine receptor 4 (CXCR4), plerixafor mobilizes PBSC more readily and is also well-tolerated. Studies of autologous and allogeneic transplantation of plerixafor-mobilized grafts have shown prompt and stable engraftment (Cashen et al, 2007).
Flomenberg et al (2005) tested the hypotheses that the combination of AMD3100 plus G-CSF (A + G) would be superior to G-CSF alone (G) in mobilizing hematopoietic progenitor cells (HPC) and that A + G-mobilized cells would engraft as well as G-mobilized cells. The primary objective was to determine whether patients mobilized more progenitor cells per unit of blood volume of apheresis after A + G administration versus G alone. Secondary objectives were to determine whether patients mobilized with A + G compared with G alone required fewer apheresis procedures to reach the target level of at least 5 x 10(6) CD34(+) cells/kg for transplant and to determine whether patients mobilized with A + G had at least a 90 % success rate of autologous transplantation as indexed by neutrophil engraftment by day 21. Each patient served as his or her own control in a sequential mobilization design. All study objectives were met without significant toxicity. The results demonstrated that the combination of A + G is generally safe, effective, and superior to G alone for autologous HPC mobilization.
In a phase II clinical trial, Gazitt and associates (2007) evaluated the effect of AMD3100 on the mobilization of CD34+ cells, dendritic cells (DC) and lymphoma cells in patients with refractory or relapsed non-Hodgkin's lymphoma (NHL). A total of 11 patients received 16 microg/kg daily of G-CSF for 4 days followed by 240 microg/kg of AMD3100 given subcutaneously on a new schedule of 9 to 10 hours before apheresis collection on day 5. Administration of G-CSF and AMD3100 were continued daily until greater than or equal to 2 x 10(6) CD34+ cells/kg were collected. Adequate collection of the target of CD34+ cells was achieved in all but 1 patient within 2 days, and 10/11 patients were transplanted within 2 months. All transplanted patients engrafted with a mean of 10 and 12 days for neutrophils and platelets, respectively. Addition of AMD3100 to G-CSF resulted with greater than 2.5-fold increase in CD34+ cells/microl (p = 0.0001) and in a greater than 2-fold increase in precursor dendritic cell type 1 and precursor dendritic cell type 2 cells/microl (p = 0.003). Adverse events related to AMD3100 were minimal. AMD3100 was generally safe and improved PBSC and DC cell mobilization with no apparent contamination of lymphoma cells.
Cashen and colleagues (2008) examined the safety and effectiveness of hematopoietic stem cell mobilization with plerixafor in patients with Hodgkin lymphoma (HL). A total of 22 patients with HL who were candidates for autologous stem cell transplantation (ASCT) underwent hematopoietic stem cell mobilization with a combination of G-CSF, 10 microg/kg daily, and plerixafor, 240 microg/kg subcutaneous, 10 to 11 hours prior to apheresis. The primary endpoint was the proportion of patients who collected greater than or equal to 5 x 10(6) CD34+ cells/kg. Outcomes were compared to a historical control population of HL patients mobilized with G-CSF alone. Pharmacokinetic (PK) analysis of plerixafor was performed in a subset of patients. Fifteen patients (68 %) collected greater than or equal to 5 x 10(6) CD34+ cells/kg, and 21 patients (95 %) achieved the minimum collection of greater than or equal to 2 x 10(6) CD34+ cells/kg, in a median 2 apheresis procedures. Both the proportion of patients collecting greater than or equal to 5 x 10(6) CD34+ cells/kg and the median CD34+ cells collected in days 1 to 2 of apheresis were significantly improved over historical controls. The PK of plerixafor in this patient population was similar to that previously seen in healthy volunteers. The regimen was generally safe and well-tolerated.
Calandra and colleagues (2008) reported that AMD3100 plus G-CSF can successfully mobilize CD34+ cells from NHL, HL and multiple myeloma (MM) patients previously failing mobilization with chemotherapy and/or cytokine treatment. A cohort of 115 data-audited poor mobilizers was assessed, with the objective being to collect greater than or equal to 2 x 10(6) CD34+ cells per kg following AMD3100 plus G-CSF mobilization. The rates of successful CD34+ cell collection were similar for patients who previously failed chemotherapy mobilization or cytokine-only mobilization: NHL -- 60.3 %, MM -- 71.4 % and HL -- 76.5 %. Following transplantation, median times to neutrophil and platelet engraftment were 11 days and 18 days, respectively. Engraftment was durable. There were no drug-related serious adverse events. Of the adverse events considered related to AMD3100, 2 (1.6 %) were severe (headache, n = 1; nightmares, n = 1). Other AMD3100-related adverse events were mild (84.8 %) or moderate (13.6 %). The most common AMD3100-related adverse events were paresthesias, gastrointestinal reactions, and injection site reactions.
Devine et al (2008) discussed their findings on rapid mobilization of functional donor hematopoietic cells without G-CSF using AMD3100. Donors (n = 25) were treated with AMD3100 at a dose of 240 mug/kg by subcutaneous injection, and leukapheresis was then initiated just 4 hours later. Two-thirds of the donors collected an allograft with a CD34(+) cell dose sufficient for transplantation after just 1 dose of AMD3100. No donor experienced more than grade 1 toxicity. After a myeloablative regimen, 20 patients with hematologic malignancies received allografts collected after AMD3100 alone. All patients engrafted neutrophils (median day 10) and platelets (median day 12) promptly. Acute GVHD grades 2 through 4 occurred in 35 % of patients; 1 patient died of complications related to acute GVHD. No unexpected adverse events were observed in any of the recipients. All 14 patients surviving in remission have robust trilineage hematopoiesis and are transfusion-free with a median follow-up of 277 days (range of 139 to 964 days).
Pusic et al (2008) examined historic institutional autologous stem cell mobilization practices and assessed factors influencing mobilization failure and kinetics. In this retrospective study, these researchers analyzed clinical records of 1834 patients who underwent stem cell mobilization for autologous transplantation from November 1995 to October 2006 at the Washington University in St. Louis. Successful mobilization was defined as collection of greater than or equal to 2 x 10(6) CD34(+) cells/kg. From 1834 consecutive patients, 1040 met the investigators' inclusion criteria (502 NHL, 137 HL, and 401 MM). A total of 976 patients received G-CSF and 64 received G-CSF plus chemotherapy (G/C) for the initial mobilization. Although the median CD34(+) cell yield was higher in G/C group than in G-CSF alone group, the failure rates were similar: 18.8 % and 18.6 %, respectively. Overall, 53 % of patients collected greater than or equal to 2 x 10(6) CD34(+) cells/kg during the first apheresis with either mobilization regimen. Regardless of mobilization regimen used, MM patients had the highest total CD34(+) cell yield and required less aphereses to collect greater than or equal to 2 x 10(6) CD34(+) cells/kg. Mobilized, pre-apheresis, peripheral blood CD34(+) count correlated with first day apheresis yield (r = 0.877, p < 0.001) and 20 cells/microl was the minimum threshold needed for a successful day 1 collection. For the re-mobilization analysis, these investigators included patients from the whole database. A total of 269 of 1834 patients underwent re-mobilization using G/C, G-CSF, and/or GM-CSF, and G-CSF plus plerixafor. Only 23 % of re-mobilized patients achieved greater than or equal to 2 x 10(6) CD34(+) cells/kg and 29.7 % failed to pool sufficient number of stem cells from both collections. Patients receiving G-CSF plus plerixafor had lowest failure rates, p = 0.03. Non-Hodgkin's lymphoma patients re-mobilized with G-CSF who waited greater than or equal to 25 days before re-mobilization had lower CD34(+) cell yield than those who waited greater than or equal to 16 days, p = 0.023. Current mobilization regimens are associated with a substantial failure rate irrespective of underlying disease. Patients who fail initial mobilization are more likely to fail re-mobilization. These findings suggested that there is a need for more effective first-line mobilization agents.
On December 18, 2008, the U.S. Food and Drug Administration (FDA) approved Mozobil (plerixafor injection) to be used in combination with G-CSF for the treatment of adults with MM or NHL. The FDA approval is based on the findings of two clinical studies: one in patients with MM, the other with NHL. In these two placebo-controlled clinical trials (Studies 1 and 2, Genzyme, 2008), patients were randomized to receive either plerixafor 0.24 mg/kg or placebo on each evening prior to apheresis. Patients received daily morning doses of G-CSF 10 micrograms/kg for 4 days prior to the first dose of plerixafor or placebo and on each morning prior to apheresis. In Study 1, a total of 298 NHL patients were included in the primary effectiveness analyses. The mean age was 55.1 years (range of 29 to 75) and 57.5 years (range of 22 to 75) in the plerixafor and placebo groups, respectively, and 93 % of subjects were Caucasian. In Study 2, a total of 302 MM patients were included in the primary effectiveness analyses. The mean age was 58.2 years (range of 28 to 75) and 58.5 years (range of 28 to 75) in the plerixafor and placebo groups, respectively, and 81 % of subjects were Caucasian.
In Study 1, 59 % of NHL patients who were mobilized with plerixafor and G-CSF collected greater than or equal to 5 x 10(6) CD34+ cells/kg from the peripheral blood in 4 or fewer apheresis sessions, compared with 20 % of patients who were mobilized with placebo and G-CSF (p < 0.001). Other CD34+ cell mobilization outcomes showed similar findings. The median number of days to reach greater than or equal to 5 x 10(6) CD34+ cells/kg was 3 days for the plerixafor group and not evaluable for the placebo group.
In Study 2, 72 % of MM patients who were mobilized with plerixafor and G-CSF collected greater than or equal to 6 x 10(6) CD34+ cells/kg from the peripheral blood in 2 or fewer apheresis sessions, compared with 34 % of patients who were mobilized with placebo and G-CSF (p < 0.001). Other CD34+ cell mobilization outcomes showed similar findings. The median number of days to reach greater than or equal to 6 x 10(6) CD34+ cells/kg was 1 day for the plerixafor group and 4 days for the placebo group. Multiple factors can influence time to engraftment and graft durability following stem cell transplantation. For transplanted patients in the phase 3 studies, time to neutrophil and platelet engraftment and graft durability were similar across the treatment groups.
Common side effects reported in these clinical trials and other smaller studies include diarrhea, fatigue, headaches, nausea, joint pain, injection site reactions, as well as dizziness and vomiting.
Flomenberg and colleagues (2010) evaluated the safety and efficacy of plerixafor as a single agent when given subcutaneously and followed by apheresis 6 hours later for the mobilization of hematopoietic stem cells (HSC) for transplantation in 9 patients with MM. All patients mobilized enough cells for at least 1 transplant, and demonstrated prompt recovery of hematopoietic function. Median time to engraftment was 10.5 days for neutrophils and 21 days for platelets. Significant adverse events were not observed. Recovery of peripheral blood cell counts was durable in all surviving patients. Despite these successes, mobilization with plerixafor alone was modest. However, in clinical circumstances where G-CSF or chemotherapy based-mobilization should not be used, mobilization with plerixafor alone may be required and effective. The authors concluded that further research into single agent use should focus on alternate route of administration as well as adjustment of the timing of the apheresis to improve HSC yield.
WHIM (warts, hypogammaglobulinemia, infections, and myelokathexis) syndrome, a rare congenital immunodeficiency disorder characterized by chronic non-cyclic neutropenia, is caused by autosomal dominant mutations in the gene for the chemokine receptor, CXCR4, resulting in a carboxy-terminus truncation of the receptor of between ten and 19 residues. This condition is one of only a few diseases directly and primarily caused by an aberrant chemokine. Patients with WHIM syndrome exhibit increased susceptibility to bacterial and viral infections, especially from common serotype human papilloma virus, resulting in warts on the hands and feet starting in childhood. Myelokathexis refers to retention of neutrophils in the bone marrow. Furthermore, lymphocytes and gammaglobulins are often deficient.
Dale and colleagues (2011) stated that mutations in CXCR4 cause severe leukopenia in myelokathexis or WHIM syndrome. Plerixafor inhibits binding of CXCR4 to its ligand CXCL12. These researchers investigated the effects of plerixafor (0.04 to 0.24 mg/kg body weight) administered at 2 to 4 day intervals in 6 patients. Outcome measures were the patients' complete blood cell counts, CD34+ cell counts and lymphocyte subtypes compared with 5 normal subjects similarly treated with plerixafor. All patients showed prompt leukocytosis with maximum blood neutrophils and lymphocytes at 6 to 12 hrs. Blood neutrophils peaked at 6 to 12 hrs, increasing from a mean baseline of 0.4 +/- 0.1 x 10(9)/L, to mean peak of 4.5 +/- 0.78 x 10(9)/L. Lymphocytes also increased; the greatest increase was in B cells (CD19+ cells), a greater than 40-fold increase over baseline at the 0.08 mg/kg dose. None of the patients experienced any significant adverse effects. The authors concluded that plerixafor is a promising therapy for this condition.
Burger and associates (2011) stated that despite advances in surgery, chemotherapy and radiotherapy over the last decades, the death rate from lung cancer has remained largely unchanged, which is mainly due to metastatic disease. Because of the overall poor prognosis, new treatment strategies for lung cancer patients are urgently needed, and targeting CXCR4 constitutes such a novel, attractive strategy. Tumor cell migration and metastasis share many similarities with leukocyte trafficking, which is critically regulated by chemokine receptors and adhesion molecules. Lung cancer cells express CXCR4 (CD184), a seven-transmembrane G-protein-coupled chemokine receptor. Stromal cells within the tumor microenvironment constitutively secrete stromal cell-derived factor-1 (SDF-1/CXCL12), the ligand for CXCR4. Activation of CXCR4 induces lung cancer cell migration and adhesion to stromal cells, which in turn provides growth- and drug-resistance signals to the tumor cells. CXCR4 antagonists, such as plerixafor (AMD3100) and T140 analogs (TN14003/BKT140), can disrupt CXCR4-mediated tumor cell adhesion to stromal cells and sensitize lung cancer cells to cytotoxic drugs. Thus, targeting the CXCR4-CXCL12 axis is a novel, attractive therapeutic approach in small-cell lung cancer and non-small-cell lung cancer. The authors summarized data about the cellular and molecular microenvironment in small-cell lung cancer and non-small-cell lung cancer, as well as the role of CXCR4 in tumor-stroma cross-talk. In addition, they reviewed the current status of the pre-clinical and clinical development of CXCR4 antagonists.
Uy and colleagues (2012) stated that the interaction of acute myeloid leukemia (AML) blasts with the leukemic micro-environment is postulated to be an important mediator of resistance to chemotherapy and disease relapse. These investigators hypothesized that inhibition of the CXCR4/CXCL12 axis by plerixafor would disrupt the interaction of leukemic blasts with the environment and increase the sensitivity of AML blasts to chemotherapy. In a phase I/II study, 52 patients with relapsed or refractory AML were treated with plerixafor in combination with mitoxantrone, etoposide, and cytarabine. In phase I, plerixafor was escalated to a maximum of 0.24 mg/kg/day without any dose-limiting toxicities. In phase II, 46 patients were treated with plerixafor 0.24 mg/kg/day in combination with chemotherapy with an overall complete remission and complete remission with incomplete blood count recovery rate (CR + CRi) of 46 %. Correlative studies demonstrated a 2-fold mobilization in leukemic blasts into the peripheral circulation. No evidence of symptomatic hyper-leukocytosis or delayed count recovery was observed with the addition of plerixafor. The authors concluded that the addition of plerixafor to cytotoxic chemotherapy is feasible in AML, and resulted in encouraging rates of remission with correlative studies demonstrating in-vivo evidence of disruption of the CXCR4/CXCL12 axis. These preliminary findings need to be validated by well-designed studies.
Stamatopoulos and co-workers (2012) the ability of plerixafor (AMD3100), a CXCR4 antagonist, to sensitize chronic lymphocytic leukemia (CLL) cells to chemotherapy in a CLL/mesenchymal stromal cell based or nurse-like cell based micro-environment co-culture model. Plerixafor decreased CXCR4 expression signal (n = 15, p = 0.0078) and inhibited actin polymerization/migration in response to SDF-1α (n = 8, p < 0.01) and pseudo-emperipolesis (n = 10, p = 0.0010), suggesting that AMD3100 interferes with CLL cell trafficking. Plerixafo did not have a direct effect on apoptosis when CLL cells were cultured alone (n = 10, p = 0.8812). However, when they were cultured with SDF-1α, mesenchymal stromal cells or nurse-like cells (protecting them from apoptosis, p < 0.001), CLL cell pre-treatment with AMD3100 significantly inhibited these protective effects (n = 8, p < 0.01) and decreased the expression of the anti-apoptotic proteins MCL-1 and FLIP. Furthermore, combining AMD3100 with various drugs (fludarabine, cladribine, valproïc acid, bortezomib, flavopiridol, methylprednisolone) in the authors' mesenchymal stromal cell co-culture model enhanced drug-induced apoptosis (n = 8, p < 0.05) indicating that AMD3100 could mobilize CLL cells away from their protective micro-environment, making them more accessible to conventional therapies. The authors concluded that taken together, these data demonstrated that interfering with the SDF-1α/CXCR4 axis by using plerixafor inhibited CLL cell trafficking and micro-environment-mediated protective effects. They stated that combining plerixafor with other drugs may, therefore, represent a promising therapeutic approach to kill CLL cells.
Agarwal and colleagues (2012) stated that sequestration in the bone marrow niche may allow leukemic stem cells (LSC) to evade exposure to drugs. Since the CXCR4/SDF-1 axis is an important mechanism of LSC interaction with marrow stroma, these researchers examined if plerixafor may dislodge chronic myeloid leukemia (CML) cells from the niche, sensitizing them to tyrosine kinase inhibitors (TKIs). They initially treated mice with retrovirally induced CML-like disease with imatinib plus plerixafor. Plerixafor mobilized CXCR4-positive cells, but no difference was observed in leukemia burden, possibly reflecting insufficient disease control by imatinib. In a 2nd series of experiments, these investigators tested the combination of plerixafor with dasatinib in the same as well as an attenuated CML model. Despite much improved leukemia control, plerixafor failed to reduce leukemia burden over dasatinib alone. Additionally mice receiving plerixafor had an increased incidence of neurological symptoms in association with central nervous system (CNS) infiltration by BCR-ABL expressing cells. The authors concluded that plerixafor is ineffective in reducing leukemia burden in this model, but promotes CNS infiltration.
Vives et al (2012) noted that some malignant tumors in childhood require high-dose chemotherapy with stem cell support to achieve a cure. In patients heavily pre-treated with myelosuppressive chemotherapy or irradiation, G-CSF may fail to mobilize stem cells from the bone marrow. Based on the experience with lymphoma and myeloma patients in whom PBSC collection following mobilization with G-CSF failed, these researchers successfully employed plerixafor in a 14-year old female diagnosed with Ewing's sarcoma in early relapse treated with 3 lines of chemotherapy in whom PBSC could not be mobilized using either G-CSF alone or G-CSF following chemotherapy. No side effects were observed. The authors stated that plerixafor may be an effective and safe agent for stem cell collection in pediatric patients with solid tumors, although new studies are needed to evaluate its safety and effectiveness.
Worel and colleagues (2012) stated that plerixafor with G-CSF has been shown to enhance stem cell mobilization in patients with multiple myeloma and lymphoma with previous mobilization failure. These investigators reported the experience in insufficiently mobilizing patients diagnosed with non-hematologic diseases. A total of 33 patients with germ cell tumor (n = 11), Ewing sarcoma (n = 6), Wiscott-Aldrich disease (n = 5), neuroblastoma (n = 4), and other non-hematologic diseases (n = 7) were included in the study. Plerixafor was limited to patients with previous or current stem cell mobilization failure and given after 4 days of G-CSF (n = 21) or after chemotherapy and G-CSF (n = 12) in patients who mobilized poorly. Overall, 28 (85 %) patients succeeded in collecting at least 2 × 10(6) /kg body weight (b.w.) CD34+ cells (median, 5.0 × 10(6) /kg b.w. CD34+ cells; range of 2.0 × 10(6) to 29.5 × 10(6) /kg b.w. CD34+ cells), and 5 (15 %) patients collected a median of 1.5 × 10(6) /kg b.w. CD34+ cells (range of 0.9 × 10(6) to 1.8 × 10(6) /kg b.w. CD34+ cells). Nineteen patients proceeded to transplantation. The median dose of CD34+ cells infused was 3.3 × 10(6)/kg b.w. (range of 2.3 × 10(6) to 6.7 × 10(6) /kg b.w. CD34+ cells). The median numbers of days to neutrophil and platelet engraftment were 11 (range of 9 to 12) and 15 (range of 10 to 25) days, respectively. The authors concluded that these findings emphasized the role of plerixafor in combination with G-CSF or chemotherapy and G-CSF as an effective mobilization regimen with the potential of successful stem cell collection. Accordingly, plerixafor seems to be safe and effective in patients with non-hematologic diseases. However, they stated that larger prospective studies are needed to further evaluate its use in these patients.