Relevance and Clinical Implications of Tumor Cell Mobilization in the Autologous Transplant Setting

Stem cell mobilization in multiple myeloma: challenges, strategies, and current developments

Among hematological malignancies, multiple myeloma (MM) represents the leading indication of autologous hematopoietic stem cell transplantation (auto-HCT). Auto-HCT is predominantly performed with peripheral blood stem cells (PBSCs), and the mobilization and collection of PBSCs are essential steps for auto-HCT. Despite the improved success of conventional methods with the incorporation of novel agents for PBSC mobilization in MM, mobilization failure is still a concern. The current review comprehensively summarizes various mobilization strategies for mobilizing PBSCs in MM patients and the evolution of these strategies over time. Moreover, existing evidence substantiates that the mobilization regimen used may be an important determinant of graft content. However, limited data are available on the effects of graft characteristics in patient outcomes other than hematopoietic engraftment. In this review, we discussed the effect of graft characteristics on clinical outcomes, mobilization failure, factors predictive of poor mobilization, and potential mobilization regimens for such patients.

Benefits of plerixafor for mobilization of peripheral blood stem cells prior to autologous transplantation: a dual-center retrospective cohort study

BACKGROUND AIMS

Before autologous stem cell transplantation (ASCT), hematopoietic stem cells must be stimulated to move from the bone marrow to the peripheral blood for harvesting. Plerixafor, a C-X-C chemokine receptor type 4 antagonist, is used to increase stem cell harvests. However, the effects of plerixafor on post-ASCT outcomes remain unclear.

METHODS

In a dual-center retrospective cohort study of 43 Japanese patients who received ASCT, the authors compared transplantation outcomes in patients who underwent stem cell mobilization with granulocyte colony-stimulating factor with (n = 25) or without (n = 18) plerixafor.

RESULTS

The number of days to neutrophil and platelet engraftment was significantly shorter with plerixafor than without plerixafor, as assessed by univariate (neutrophil, P = 0.004, platelet, P = 0.002), subgroup, propensity score matching and inverse probability weighting analyses. Although the cumulative incidence of fever was comparable with or without plerixafor (P = 0.31), that of sepsis was significantly lower with plerixafor than without (P < 0.01). Thus, the present data indicate that plerixafor leads to earlier neutrophil and platelet engraftment and a reduction of infectious risk.

CONCLUSIONS

The authors conclude that plerixafor may be safe to use and that it reduces the risk of infection in patients with a low CD34+ cell count the day before apheresis.

[Analysis of the efficacy and safety of plerixafor combined with G-CSF in plasma cell disease mobilization]

目的

分析普乐沙福联合G-CSF在浆细胞疾病自体造血干细胞中动员的效果及安全性。

方法

回顾性分析2018年1月至2019年12月在北京大学人民医院使用普乐沙福联合G-CSF进行自体造血干细胞动员的浆细胞疾病患者的基线临床资料、采集成功率及不良反应。

结果

共纳入49例浆细胞疾病患者，多发性骨髓瘤（MM）39例（79.6%），淀粉样变性8例（16.3%），肾脏意义的单克隆免疫球蛋白沉积病（MGRS）2例（4.1%），肾功能不全16例（32.7%）。其他动员方案既往采集失败患者7例（14.3%）。使用普乐沙福动员后，中位采集次数1（1～3）次，中位采集天数2（1～3）d，一次采集成功47例（95.9%），两次采集累积成功率为100%。在16例肾功能不全的患者中，5例（31.3%）患者第1天采集成功，8例（50%）需要第2天采集，3例（18.8%）需要第3天采集。主要不良反应依次为乏力、失眠、腹痛、腹泻、头晕、关节痛。共37例患者行auto-HSCT，白细胞中位植活时间11（8～13）d，血小板中位植活时间11（9～26）d。

结论

普乐沙福联合G-CSF用于浆细胞疾病患者自体造血干细胞动员成功率高，不良反应少，即使在肾功能不全的患者中也具有较高的动员成功率。

Getting blood out of a stone: Identification and management of patients with poor hematopoietic cell mobilization

Hematopoietic cell transplantation (HCT) has become a primary treatment for many cancers. Nowadays, the primary source of hematopoietic cells is by leukapheresis collection of these cells from peripheral blood, after a forced egress of hematopoietic cells from marrow into blood circulation, a process known as "mobilization". In this process, mobilizing agents disrupt binding interactions between hematopoietic cells and marrow microenvironment to facilitate collection. As the first essential step of HCT, poor mobilization, i.e. failure to obtain a desired or required number of hematopoietic cell, is one of the major factors affecting engraftment or even precluding transplantation. This review summarizes the available mobilization regimens using granulocyte-colony stimulating factor (G-CSF) and plerixafor, as well as the current understanding of the factors that are associated with poor mobilization. Strategies to mobilize patients or healthy donors who failed previous mobilization are discussed. Multiple novel agents are under investigation and some of them have shown the potential to enhance the mobilization response to G-CSF and/or plerixafor. Further investigation of the risk factors including genetic factors will offer an opportunity to better understand the molecular mechanism of mobilization and help develop new therapeutic strategies for successful mobilizations.

A Pilot, Exploratory, Randomized, Phase II Safety Study Evaluating Tumor Cell Mobilization and Apheresis Product Contamination in Patients Treated with Granulocyte Colony-Stimulating Factor Alone or Plus Plerixafor

Because of the potential risk of tumor cell mobilization with granulocyte colony-stimulating factor (G-CSF), it is crucial to evaluate any potential effect of plerixafor treatment in the presence of G-CSF on multiple myeloma (MM) cell mobilization. This was an open-label, multicenter, randomized, exploratory, safety study (NCT01753453) that investigated the extent of MM cell mobilization after treatment with G-CSF + plerixafor in patients who were deemed poor mobilizers of hematopoietic stem cells. The primary efficacy outcome was the number of MM cells in peripheral blood and apheresis product after G-CSF + plerixafor treatment versus G-CSF alone. Key secondary efficacy outcomes included overall survival and disease status up to 2 years after the first G-CSF dose. Twenty patients were randomized and received at least 1 dose of study treatment. There were no patients with MM cells in peripheral blood up to day 8 G-CSF administration in either treatment group. Up to day 8 no patient in the G-CSF + plerixafor arm and only 1 patient in the G-CSF arm mobilized at least 4.5 × 10⁵ MM cells in the apheresis product. Nine of 10 patients from each treatment arm proceeded to transplantation. MM cells were detected in 5 patients from each treatment arm before and after transplantation. Adverse events observed in the G-CSF + plerixafor arm were consistent with the known safety profile of plerixafor. No MM cells were detected in peripheral blood of either treatment group up to day 8 of mobilization. Only 1 patient in the G-CSF alone group mobilized at least 4.5 × 10⁵ MM tumor cells in apheresis product up to day 8. However, 50% of patients in both treatment arms had detectable amounts of MM cells in their peripheral blood pre- and post-transplantation. There were no new safety concerns with plerixafor.

Plerixafor Plus Granulocyte Colony-Stimulating Factor for Patients with Non-Hodgkin Lymphoma and Multiple Myeloma: Long-Term Follow-Up Report

The purpose of this report is to analyze long-term clinical outcomes of patients exposed to plerixafor plus granulocyte colony-stimulating factor (G-CSF) for stem cell mobilization. This was a study of patients with non-Hodgkin lymphoma (NHL; n = 167) and multiple myeloma (MM; n = 163) who were enrolled in the long-term follow-up of 2 pivotal phase III studies (NCT00741325 and NCT00741780) of 240 µg/kg plerixafor plus 10 µg/kg G-CSF, or placebo plus 10 µg/kg G-CSF to mobilize and collect CD34⁺ cells for autologous hematopoietic stem cell transplantation. Overall survival (OS) and progression-free survival (PFS) were evaluated over a 5-year period following the first dose of plerixafor or placebo. The probability of OS was not significantly different in patients with NHL or MM treated with plerixafor or placebo (NHL: 64%; 95% confidence interval [CI], 56% to 71% versus 56%; 95% CI, 44% to 67%, respectively; MM: 64%; 95% CI, 54% to 72% versus 64%; 95% CI, 53% to 73%, respectively). In addition, there was no statistically significant difference in the probability of PFS over 5 years between treatment groups in patients with NHL (50%; 95% CI, 44% to 67% for plerixafor versus 43%; 95% CI, 31% to 54% for placebo) or those with MM (17%; 95% CI, 10% to 24% for plerixafor versus 30%; 95% CI, 21% to 40% for placebo). In this long-term follow-up study, the addition of plerixafor to G-CSF for stem cell mobilization did not affect 5-year survival in patients with NHL or patients with MM.

Management of mobilization failure in 2017

In contemporary clinical practice, almost all allogeneic transplantations and autologous transplantations now capitalize on peripheral blood stem cells (PBSCs) as opposed to bone marrow (BM) for the source of stem cells. In this context, granulocyte colony-stimulating factor (G-CSF) plays a pivotal role as the most frequently applied frontline agent for stem cell mobilization. For patients classified as high-risk, chemotherapy based mobilization regimens can be preferred as a first choice and it is notable that this also used for remobilization. Mobilization failure occurs at a rate of 10%-40% with traditional strategies and it typically leads to low-efficiency practices, resource wastage, and delayed in treatment intervention. Notably, however, several factors can impact the effectiveness of CD34⁺ progenitor cell mobilization, including patient age and medical history (prior chemotherapy or radiotherapy, disease and marrow infiltration at the time of mobilization). In recent years, main (yet largely ineffective) approach was to increase G-CSF dose and add SCF, but novel and promising pathways have been opened up by the synergistic impact of a reversible inhibitor of CXCR4, plerixafor, with G-CSF. The literature shows to its favorable results in upfront and failed mobilizers, and it is necessary to use plerixafor (or equivalent agents) to optimize HSC harvest in poor mobilizers. Different CXCR4 inhibitors, growth hormone, VLA4 inhibitors, and parathormone, have been cited as new agents for mobilization failure in recent years. In view of the above considerations, the purpose of this paper is to examine the mobilization of PBSC while focusing specifically on poor mobilizers.

Preemptive plerixafor injection added to pegfilgrastim after chemotherapy in non-Hodgkin lymphoma patients mobilizing poorly

Filgrastim is usually combined with chemotherapy to mobilize hematopoietic progenitor cells in non-Hodgkin lymphoma (NHL) patients. Limited information is available on the efficacy of a preemptive plerixafor (PLER) injection in poor mobilizers after chemotherapy and pegfilgrastim. In this prospective study, 72 patients with NHL received chemotherapy plus pegfilgrastim, and 25 hard-to-mobilize patients received also PLER. The usefulness and efficacy of our previously developed algorithm for PLER use in pegfilgrastim-containing mobilization regimen were evaluated as well as the graft cellular composition, hematological recovery, and outcome after autologous stem cell transplantation (auto-SCT) according to the PLER use. A median 3.4-fold increase in blood CD34⁺ cell counts was achieved after the first PLER dose. The minimum collection target was achieved in the first mobilization attempt in 66/72 patients (92%) and 68 patients (94%) proceeded to auto-SCT. An algorithm for PLER use was fulfilled in 76% of the poor mobilizers. Absolute numbers of T-lymphocytes and NK cells were significantly higher in the PLER group, whereas the number of CD34⁺ cells collected was significantly lower. Early neutrophil engraftment was slower in the PLER group, otherwise hematological recovery was comparable within 12 months from auto-SCT. No difference was observed in survival according to the PLER use. Chemotherapy plus pegfilgrastim combined with preemptive PLER injection is an effective and convenient approach to minimize collection failures in NHL patients intended for auto-SCT. A significant effect of PLER on the graft cellular composition was observed, but no difference in outcome after auto-SCT was detected.

An Unusual Cause of Pulmonary Nodules After Stem Cell Transplantation

A man in his 20s with a history of classical Hodgkin's lymphoma was admitted with fever. His original lymphoma diagnosis was made 3 years prior, when he had presented with lymphadenopathy and a mediastinal mass. He had relapsed disease despite chemotherapy and radiation. As a result, he underwent autologous peripheral blood stem cell transplant (SCT) 6 months prior to current presentation and subsequently allogeneic SCT 2 months prior for added graft vs tumor effect.

Multigene Measurable Residual Disease Assessment Improves Acute Myeloid Leukemia Relapse Risk Stratification in Autologous Hematopoietic Cell Transplantation

We report here the largest study to date of adult patients with acute myeloid leukemia (AML) tested for measurable residual disease (MRD) at the time of autologous hematopoietic cell transplantation (auto-HCT). Seventy-two adult patients who underwent transplantation between 2004 and 2013 at a single academic medical center (University of California San Francisco) were eligible for this retrospective study based on availability of cryopreserved granulocyte colony-stimulating factor (GCSF)-mobilized autologous peripheral blood progenitor cell (PBPC) leukapheresis specimens ("autografts"). Autograft MRD was assessed by molecular methods (real-time quantitative PCR [RQ-PCR] for Wilms tumor 1 (WT1) alone or a multigene panel) and by multiparameter flow cytometry (MPFC). WT1 RQ-PCR testing of the autograft had low sensitivity for relapse prediction (14%) and a negative predictive value of 51%. MPFC failed to identify MRD in any of 34 autografts tested. Combinations of molecular MRD assays, however, improved prediction of post-auto-HCT relapse. In multivariate analysis of clinical variables, including age, gender, race, cytogenetic risk category, and CD34⁺ cell dose, only autograft multigene MRD as assessed by RQ-PCR was statistically significantly associated with relapse. One year after transplantation, only 28% patients with detectable autograft MRD were relapse free, compared with 67% in the MRD-negative cohort. Multigene MRD, while an improvement on other methods tested, was however suboptimal for relapse prediction in unselected patients, with specificity of 83% and sensitivity of 46%. In patients with known chromosomal abnormalities or mutations, however, better predictive value was observed with no relapses observed in MRD-negative patients in the first year after auto-HCT compared with 83% incidence of relapse in the MRD-positive patients (hazard ratio, 12.45; P = .0016). In summary, increased personalization of MRD monitoring by use of a multigene panel improved the ability to risk stratify patients for post-auto-HCT relapse. WT1 RQ-PCR and flow cytometric assessment for AML MRD in autograft samples had limited value for predicting relapse after auto-HCT. We demonstrate that cryopreserved autograft material presents unique challenges for AML MRD testing because of the masking effects of previous GCSF exposure on gene expression and flow cytometry signatures. In the absence of information regarding diagnostic characteristics, sources other than GCSF-stimulated PBSC leukapheresis specimens should be considered as alternatives for MRD testing in AML patients undergoing auto-HCT.

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.

The ex vivo purge of cancer cells using oncolytic viruses: recent advances and clinical implications

Hematological malignancies are treated with intensive high-dose chemotherapy, with or without radiation. This is followed by hematopoietic stem cell (HSC) transplantation (HSCT) to rescue or reconstitute hematopoiesis damaged by the anticancer therapy. Autologous HSC grafts may contain cancer cells and purging could further improve treatment outcomes. Similarly, allogeneic HSCT may be improved by selectively purging alloreactive effector cells from the graft rather than wholesale immune cell depletion. Viral agents that selectively replicate in specific cell populations are being studied in experimental models of cancer and immunological diseases and have potential applications in the context of HSC graft engineering. This review describes preclinical studies involving oncolytic virus strains of adenovirus, herpes simplex virus type 1, myxoma virus, and reovirus as ex vivo purging agents for HSC grafts, as well as in vitro and in vivo experimental studies using oncolytic coxsackievirus, measles virus, parvovirus, vaccinia virus, and vesicular stomatitis virus to eradicate hematopoietic malignancies. Alternative ex vivo oncolytic virus strategies are also outlined that aim to reduce the risk of relapse following autologous HSCT and mitigate morbidity and mortality due to graft-versus-host disease in allogeneic HSCT.

Granulocyte colony-stimulating factor off-target effect on nerve outgrowth promotes prostate cancer development

The hematopoietic growth factor granulocyte colony-stimulating factor (G-CSF) has a role in proliferation, differentiation and migration of the myeloid lineage and in mobilizing hematopoietic stem and progenitor cells into the bloodstream. However, G-CSF has been newly characterized as a neurotrophic factor in the brain. We recently uncovered that autonomic nerve development in the tumor microenvironment participates actively in prostate tumorigenesis and metastasis. Here, we found that G-CSF constrains cancer to grow and progress by, respectively, supporting the survival of sympathetic nerve fibers in 6-hydroxydopamine-sympathectomized mice and also, promoting the aberrant outgrowth of parasympathetic nerves in transgenic or xenogeneic prostate tumor models. This provides insight into how neurotrophic growth factors may control tumor neurogenesis and may lead to new antineurogenic therapies for prostate cancer.

Bone marrow plasma cell assessment before peripheral blood stem cell mobilization in patients with multiple myeloma undergoing autologous stem cell transplantation

The current definition of complete response (CR) in multiple myeloma (MM) includes negative serum and urine immunofixation (IFE) tests and <5% bone marrow plasma cells (BMPCs). However, many studies of the prognostic impact of pretransplant response have not included BMPCs. We evaluated the prognostic impact of BMPC assessment before peripheral blood stem cell (PBSC) mobilization on subsequent transplant outcomes. BMPCs were assessed by CD138, kappa, and lambda immunostaining in 106 patients. After a median followup of 24.5 months, patients with <5% BMPCs had a significantly better progression-free survival (PFS) compared to those with ≥5% BMPCs (P = 0.005). Patients with <5% BMPCs + serologic CR showed superior PFS compared to those with <5% BMPCs + serologic non-CR (P = 0.050) or ≥5% BMPCs + serologic non-CR (P = 0.001). Interestingly, the prognostic impact of BMPCs was more apparent for patients who did not achieve a serologic CR (P = 0.042) compared to those with a serologic CR (P = 0.647). We concluded that IFE negativity and <5% BMPCs before PBSC mobilization were important factors to predict PFS in patients with MM undergoing ASCT. Particularly, a significant impact of <5% BMPCs was observed in patients who did not achieve IFE negativity.

Autologous haematopoietic stem cell mobilisation in multiple myeloma and lymphoma patients: a position statement from the European Group for Blood and Marrow Transplantation

Autologous haematopoietic SCT with PBSCs is regularly used to restore BM function in patients with multiple myeloma or lymphoma after myeloablative chemotherapy. Twenty-eight experts from the European Group for Blood and Marrow Transplantation developed a position statement on the best approaches to mobilising PBSCs and on possibilities of optimising graft yields in patients who mobilise poorly. Choosing the appropriate mobilisation regimen, based on patients' disease stage and condition, and optimising the apheresis protocol can improve mobilisation outcomes. Several factors may influence mobilisation outcomes, including older age, a more advanced disease stage, the type of prior chemotherapy (e.g., fludarabine or melphalan), prior irradiation or a higher number of prior treatment lines. The most robust predictive factor for poor PBSC collection is the CD34(+) cell count in PB before apheresis. Determination of the CD34(+) cell count in PB before apheresis helps to identify patients at risk of poor PBSC collection and allows pre-emptive intervention to rescue mobilisation in these patients. Such a proactive approach might help to overcome deficiencies in stem cell mobilisation and offers a rationale for the use of novel mobilisation agents.

Plerixafor for mobilization of blood stem cells in autologous transplantation: an update

INTRODUCTION

About 99% of all autologous transplants are now performed with blood stem cells. G-CSF alone or combined with chemotherapy have been used to mobilize CD34(+) cells. Plerixafor is a novel drug used for mobilization purposes.

AREAS COVERED

We have evaluated recent data in regard to plerixafor use in predicted or proven poor mobilizers. In addition, we have looked for preemptive strategies to optimize the use of this expensive drug. Also cost-efficacy issues and effects of plerixafor on graft composition and post-transplant outcomes will be discussed.

EXPERT OPINION

Plerixafor added to G-CSF is superior than G-CSF alone for mobilization of CD34(+) cells. This combination is also efficient in patients who have failed a previous mobilization attempt with other methods or in patients with risk factors for poor mobilization. Addition of plerixafor to G-CSF or chemotherapy plus G-CSF mobilization in patients who appear to mobilize poorly is under active investigation and algorithms for a preemptive use of this expensive agent have been proposed. Grafts collected after plerixafor appear to contain more lymphoid cells than the grafts collected without it. Whether this affects post-transplant outcomes such as immune reconstitution and risk of relapse needs to be evaluated.

The incidence of tumor cell contamination of peripheral blood stem cells: a meta-analysis to evaluate the impact of mobilization regimens and the influence on outcomes in breast cancer patients

Tumor cell contamination (TCC) of peripheral blood stem cells (PBSCs) is a major risk in the autologous PBSC transplant setting. However, the effect of different mobilization regimens (cytokines only versus cytokines + chemotherapy) on TCC of PBSCs and its impact on treatment outcomes have not been systematically reviewed. In the present meta-analysis, we aimed to investigate this effect in breast cancer patients since multiple studies have been conducted in this setting. We systematically searched MEDLINE and Cochrane Library up to May 2012. Seventeen studies (1,819 patients) were assessed. There was no significant difference in the incidence of TCC of PBSCs between the two mobilization regimens. When the analysis was restricted to granulocyte colony-stimulating factor as a cytokine, this difference was again not significant. We also found that TCC of PBSCs was associated with a higher annual recurrence rate in these patients. This suggests that there may be a significant risk for reinfusion of tumor cell-positive PBSCs, and whether it can increase the risk of disease recurrence needs to be determined. This study also raises important questions regarding the causes of TCC of PBSCs. These issues should be investigated systematically in PBSC transplant patients.

Optimizing autologous stem cell mobilization strategies to improve patient outcomes: consensus guidelines and recommendations

Autologous hematopoietic stem cell transplantation (aHSCT) is a well-established treatment for malignancies such as multiple myeloma (MM) and lymphomas. Various changes in the field over the past decade, including the frequent use of tandem aHSCT in MM, the advent of novel therapies for the treatment of MM and lymphoma, and the addition of new stem cell mobilization techniques, have led to the need to reassess current stem cell mobilization strategies. Mobilization failures with traditional strategies are common and result in delays in treatment and increased cost and resource utilization. Recently, plerixafor-containing strategies have been shown to significantly reduce mobilization failure rates, but the ideal method to maximize stem cell yields and minimize costs associated with collection has not yet been determined. A panel of experts convened to discuss the currently available data on autologous hematopoietic stem cell mobilization and transplantation and to devise guidelines to optimize mobilization strategies. Herein is a summary of their discussion and consensus.

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.

Preemptive use of plerixafor in difficult-to-mobilize patients: an emerging concept

Mobilized peripheral blood (PB) is the preferred source of stem cells (PBSCs) for autologous stem cell transplantation (ASCT). The use of cytokines, alone or in combination with chemotherapy (chemomobilization), is currently the most common strategy applied to collect PBSCs. However, a significant proportion of patients with lymphoid malignancies fail to mobilize enough PBSCs to proceed to ASCT. Plerixafor has been recently introduced for clinical use to enhance PBSC mobilization and has been shown to be more effective than granulocyte-colony-stimulating factor (G-CSF) alone in patients with multiple myeloma or non-Hodgkin's lymphoma. There is limited experience on combining plerixafor with chemotherapy plus G-CSF in patients who mobilize poorly. This review attempts to summarize the published experience on the preemptive use of plerixafor after chemomobilization or G-CSF mobilization to enhance stem cell collection and to prevent mobilization failure. Current evidence suggests that addition of plerixafor is safe and effective in the large majority of the patients with low blood CD34+ cell counts after mobilization and/or poor yield after the first collection(s). Circulating CD34+ cell counts can be increased by severalfold with plerixafor and the majority of the patients considered difficult to mobilize can be successfully collected. Although more studies are needed to evaluate proper patient selection and optimal timing for the addition of plerixafor after chemotherapy, its mechanism of action inducing the rapid release of CD34+ cells from the marrow to the PB makes this molecule suitable for its "preemptive" use in patients who are difficult to mobilize.