CXCR4 inhibitors selectively eliminate CXCR4-expressing human acute myeloid leukemia cells in NOG mouse model

CXCR4 as a therapeutic target in acute myeloid leukemia

Extensive research on the CXCL12-CXCR4 axis in acute myeloid leukemia (AML) has resulted in the incorporation of novel anti-leukemia drugs targeting this axis into therapeutic strategies. However, despite this progress, a comprehensive and up-to-date review addressing the role of the CXCL12-CXCR4 axis in AML's oncogenic processes is lacking. In this review, we examine its molecular aspects influencing cancer progression, such as its impact on autonomous proliferation, apoptotic regulation, chemoresistance mechanisms, and interactions with non-leukemic cells such as MSCs and Treg cells. Additionally, we explore clinical implications, including prognosis, correlation with WBC count, blast count in the bone marrow and peripheral blood, as well as its association with FLT3-ITD, NPM1 mutations, and FAB classification. Finally, this paper extensively discusses drugs that specifically target the CXCL12-CXCR4 axis, including plerixafor/AMD3100, ulocuplumab, peptide E5, and motixafortide, shedding light on their potential therapeutic value in the treatment of AML.

Investigational CXCR4 inhibitors in early phase development for the treatment of hematological malignancies

INTRODUCTION

CXCR4/CXCL12 axis regulates cell proliferation, survival, and differentiation, as well as the homing and mobilization of hematopoietic stem cells (HSCs) from bone marrow niches to the peripheral blood. Furthermore, CXCR4 and CXCL12 are key mediators of cross-talk between hematological malignancies and their microenvironments. CXCR4 overexpression drives disease progression, boosts tumor cell survival, and promotes chemoresistance, leading to poor prognosis.

AREAS COVERED

In light of these discoveries, scientific investigations, and clinical trials have underscored the therapeutic promise found in small-molecule antagonists like plerixafor, peptides/peptidomimetics, such as BKT140, monoclonal antibodies like PF-06747143 and ulocuplumab, as well as microRNAs. Their efficacy is evident in reducing tumor burden, inducing apoptosis and sensitizing malignant cells to conventional chemotherapies. This overview delves into the pathogenic role of the CXC4/CXCL12 axis in hematological neoplasms and examines the clinical application of key CXCR4 antagonists.

EXPERT OPINION

The information collectively emphasizes the potential of CXCR4 antagonists as a therapeutic strategy for hematologic malignancies, showcasing advancements in preclinical and clinical studies. As these therapeutic strategies progress through clinical trials, their potential to reshape the prognosis of hematologic malignancies will become increasingly apparent.

CXCL12 chemokine dimer signaling modulates acute myelogenous leukemia cell migration through altered receptor internalization

Acute myeloid leukemia (AML) is a malignancy of immature myeloid blast cells with stem-like and chemoresistant cells being retained in the bone marrow through CXCL12-CXCR4 signaling. Current CXCR4 inhibitors mobilize AML cells into the bloodstream where they become more chemosensitive have failed to improve patient survival, likely reflecting persistent receptor localization on target cells. Here we characterize the signaling properties of CXCL12-locked dimer (CXCL12-LD), a bioengineered variant of the dimeric CXCL12 structure. CXCL12-LD binding resulted in lower levels of G protein, β-arrestin, and intracellular calcium mobilization, consistent with the locked dimer being a partial agonist of CXCR4. Further, CXCL12-LD failed to induce chemotaxis in AML cells. Despite these partial agonist properties, CXCL12-LD increased CXCR4 internalization compared to wildtype and locked-monomer forms of CXCL12. Analysis of a previously published AML transcriptomic data showed CXCR4 positive AML cells co-express genes involved in chemoresistance and maintenance of a blast-like state. The CXCL12-LD partial agonist effectively mobilized stem cells into the bloodstream in mice suggesting a potential role for their use in targeting CXCR4. Together, our results suggest that enhanced internalization by CXCL12-LD partial agonist signaling can avoid pharmacodynamic tolerance and may identify new avenues to better target GPCRs.

Bone Marrow Microenvironment as a Source of New Drug Targets for the Treatment of Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a heterogeneous disease with one of the worst survival rates of all cancers. The bone marrow microenvironment is increasingly being recognised as an important mediator of AML chemoresistance and relapse, supporting leukaemia stem cell survival through interactions among stromal, haematopoietic progenitor and leukaemic cells. Traditional therapies targeting leukaemic cells have failed to improve long term survival rates, and as such, the bone marrow niche has become a promising new source of potential therapeutic targets, particularly for relapsed and refractory AML. This review briefly discusses the role of the bone marrow microenvironment in AML development and progression, and as a source of novel therapeutic targets for AML. The main focus of this review is on drugs that modulate/target this bone marrow microenvironment and have been examined in in vivo models or clinically.

Combination of dociparstat sodium (DSTAT), a CXCL12/CXCR4 inhibitor, with azacitidine for the treatment of hypomethylating agent refractory AML and MDS

Leukemia stem cells utilize cell adhesion molecules like CXCR4/CXCL12 to home to bone marrow stromal niches where they are maintained in a dormant, protected state. Dociparstat sodium (DSTAT, CX-01) is a low anticoagulant heparin with multiple mechanisms of action, including inhibition of the CXCR4/CXCL12 axis, blocking HMGB1, and binding platelet factor 4 (PF-4). We conducted a pilot study adding DSTAT to azacitidine for patients with AML or MDS unresponsive to or relapsed after prior hypomethylating agent therapy, hypothesizing that DSTAT may improve response rates. Twenty patients were enrolled, with a median of 2 prior lines of therapy and 6 cycles of prior hypomethylating agents. Among fifteen patients evaluable for response, there was 1 complete remission, and 3 marrow complete remissions, for a response rate of 27 % among evaluable patients (20 % overall). Hematologic improvement was observed in 5 additional patients. The median overall survival for all enrolled patients was 205 days (95 % CI 119-302). While cytopenias and infections were common, these were not out of proportion to what would be expected in this population of patients undergoing treatment with azacitidine alone. In summary, this trial demonstrated the feasibility of combining DSTAT with azacitidine, with several responses observed, suggesting this combination warrants further study.

Novel Polyethylene Glycol-Conjugated Triazole Derivative with High Thyrointegrin αvβ3 Affinity in Acute Myeloid Leukemia Management

(1) Background: Acute myeloid leukemia (AML) accounts for up to one-third of more than 60,000 leukemia cases diagnosed annually in the U.S. Primary AML cells express membrane αvβ3 integrin, which is associated with adverse prognosis and resistance to chemotherapies. A novel anticancer compound Polyethylene glycol-conjugated bi-TriAzole Tetraiodothyroacetic acid (P-bi-TAT) interacts with high affinity (Ki 0.3 nM) and specificity with the thyrointegrin αvβ3. We evaluated P-bi-TAT activities in two different AML models representing monocytic and myelocytic forms of acute leukemia. (2) Methods and Results: The in vivo AML models were established prior to initiation of treatment protocols by grafting human leukemia cells in immunocompromised mice. IVIS imaging scans revealed that leukemic colonies were extensively established throughout the bone marrow, liver, and lung of the untreated animals. In animals treated with P-bi-TAT at daily doses ranging from 1-10 mg/kg, subcutaneously for 2-3 weeks, IVIS imaging scans revealed 95% reduction in bone marrow colonies and leukemic colonies in liver and lung. Also, the leukemic cells were not detected in bone marrow samples of P-bi-TAT-treated animals. The anti-neoplastic effect of P-bi-TAT administration on leukemic cells was associated with marked inhibition of NF-κB activity. We conclude that experimental P-bi-TAT therapy in vivo appears extraordinarily effective against the two forms of human AML models in mice. Because the P-bi-TAT molecular target, thyrointegrin αvβ3, is consistently expressed in many, if not all, clinical AML samples, P-bi-TAT-based therapy seems to have significant clinical potential in treating most AML sub-types. Hence, P-bi-TAT represents a promising targeted therapeutic agent for AML patients.

Dynamic Changes of the Bone Marrow Niche: Mesenchymal Stromal Cells and Their Progeny During Aging and Leukemia

Mesenchymal stromal cells (MSCs) are a heterogenous cell population found in a wide range of tissues in the body, known for their nutrient-producing and immunomodulatory functions. In the bone marrow (BM), these MSCs are critical for the regulation of hematopoietic stem cells (HSC) that are responsible for daily blood production and functional immunity throughout an entire organism's lifespan. Alongside other stromal cells, MSCs form a specialized microenvironment BM tissue called "niche" that tightly controls HSC self-renewal and differentiation. In addition, MSCs are crucial players in maintaining bone integrity and supply of hormonal nutrients due to their capacity to differentiate into osteoblasts and adipocytes which also contribute to cellular composition of the BM niche. However, MSCs are known to encompass a large heterogenous cell population that remains elusive and poorly defined. In this review, we focus on deciphering the BM-MSC biology through recent advances in single-cell identification of hierarchical subsets with distinct functionalities and transcriptional profiles. We also discuss the contribution of MSCs and their osteo-adipo progeny in modulating the complex direct cell-to-cell or indirect soluble factors-mediated interactions of the BM HSC niche during homeostasis, aging and myeloid malignancies. Lastly, we examine the therapeutic potential of MSCs for rejuvenation and anti-tumor remedy in clinical settings.

Role of CXCR4 in the progression and therapy of acute leukaemia

CXCR4 is expressed on leukaemia cells and haematopoietic stem cells (HSCs), and its ligand stromal-derived factor 1 (SDF-1) is produced abundantly by stromal cells in the bone marrow (BM). The SDF-1/CXCR4 axis plays important roles in homing to and retention in the protective BM microenvironment of malignant leukaemia cells and normal HSCs. CXCR4 expression is regulated by multiple mechanisms and the level of CXCR4 expression on leukaemia cells has prognostic indications in patients with acute leukaemia. CXCR4 antagonists can mobilize leukaemia cells from BM to circulation, which render them effectively eradicated by chemotherapeutic agents, small molecular inhibitors or hypomethylating agents. Therefore, such combinational therapies have been tested in clinical trials. However, new evidence emerged that drug-resistant leukaemia cells were not affected by CXCR4 antagonists, and the migration of certain leukaemia cells to the leukaemia niche was independent of SDF-1/CXCR4 axis. In this review, we summarize the role of CXCR4 in progression and treatment of acute leukaemia, with a focus on the potential of CXCR4 as a therapeutic target for acute leukaemia. We also discuss the potential value of using CXCR4 antagonists as chemosensitizer for conditioning regimens and immunosensitizer for graft-vs-leukaemia effects of allogeneic haematopoietic stem cell transplantation.

Targeting chemokines for acute lymphoblastic leukemia therapy

Acute lymphoblastic leukemia (ALL) is a hematological malignancy characterized by the malignant clonal expansion of lymphoid hematopoietic precursors. It is regulated by various signaling molecules such as cytokines and adhesion molecules in its microenvironment. Chemokines are chemotactic cytokines that regulate migration, positioning and interactions of cells. Many chemokine axes such as CXCL12/CXCR4 and CCL25/CCR9 have been proved to play important roles in leukemia microenvironment and further affect ALL outcomes. In this review, we summarize the chemokines that are involved in ALL progression and elaborate on their roles and mechanisms in leukemia cell proliferation, infiltration, drug resistance and disease relapse. We also discuss the potential of targeting chemokine axes for ALL treatments, since many related inhibitors have shown promising efficacy in preclinical trials, and some of them have entered clinical trials.

A phase I trial evaluating the effects of plerixafor, G-CSF, and azacitidine for the treatment of myelodysplastic syndromes

Interactions between the bone marrow microenvironment and MDS tumor clones play a role in pathogenesis and response to treatment. We hypothesized G-CSF and plerixafor may enhance sensitivity to azacitidine in MDS. Twenty-eight patients with MDS were treated with plerixafor, G-CSF and azacitidine with a standard 3 + 3 design. Subjects received G-CSF 10 mcg/kg D1-D8, plerixafor D4-D8, and azacitidine 75 mg/m² D4-D8, but the trial was amended to reduce G-CSF dose to 5 mcg/kg for 5 days after 2 patients had significant leukocytosis. Plerixafor was dose escalated to 560 mcg/kg/day without dose limiting toxicity. Two complete responses and 6 marrow responses were seen for an overall response rate (ORR) of 36% in evaluable patients, and ORR of 53% in patients receiving the triplet. Evidence of mobilization correlated with a higher ORR, 60% vs. 17%. Plerixafor, G-CSF and azacitidine appears tolerable when given over 5 days and has encouraging response rates.KEY POINTSPlerixafor and G-CSF can be safely combined with azacitidine for 5 days in patients with MDS.The overall response rate of 53% for evaluable patients with this regimen is higher than expected and more responses were seen in patients with blast mobilization.

At the Bedside: Profiling and treating patients with CXCR4-expressing cancers

The chemokine receptor, C-X-C chemokine receptor type 4 (CXCR4) and its ligand, C-X-C motif chemokine 12, are key mediators of hematopoietic cell trafficking. Their roles in the proliferation and metastasis of tumor cells, induction of angiogenesis, and invasive tumor growth have been recognized for over 2 decades. CXCR4 is a promising target for imaging and therapy of both hematologic and solid tumors. To date, Sanofi Genzyme's plerixafor is the only marketed CXCR4 inhibitor (i.e., Food and Drug Administration-approved in 2008 for stem cell mobilization). However, several new CXCR4 inhibitors are now being investigated as potential therapies for a variety of fluid and solid tumors. These small molecules, peptides, and Abs include balixafortide (POL6326, Polyphor), mavorixafor (X4P-001, X4 Pharmaceuticals), motixafortide (BL-8040, BioLineRx), LY2510924 (Eli Lilly), and ulocuplumab (Bristol-Myers Squibb). Early clinical evidence has been encouraging, for example, with motixafortide and balixafortide, and the CXCR4 inhibitors appear to be generally safe and well tolerated. Molecular imaging is increasingly being used for effective patient selection before, or early during CXCR4 inhibitor treatment. The use of radiolabeled theranostics that combine diagnostics and therapeutics is an additional intriguing approach. The current status and future directions for radioimaging and treating patients with CXCR4-expressing hematologic and solid malignancies are reviewed. See related review - At the Bench: Pre-Clinical Evidence for Multiple Functions of CXCR4 in Cancer. J. Leukoc. Biol. xx: xx-xx; 2020.

Leukemia Stem Cell Release From the Stem Cell Niche to Treat Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous, complex, and deadly disease, whose treatment has hardly evolved for decades and grounds on the use of intensive chemotherapy regimens. Chemotherapy helps reduce AML bulk, but promotes relapse in the long-run by selection of chemoresistant leukemia stem cells (LSC). These may diversify and result in progression to more aggressive forms of AML. In vivo models suggest that the bone marrow stem cell niche helps LSC stay dormant and protected from chemotherapy. Here, we summarize relevant changes in stem cell niche homing and adhesion of AML LSC vs. healthy hematopoietic stem cells, and provide an overview of clinical trials aiming at targeting these processes for AML treatment and future directions within this field. Promising results with various non-mutation-targeted novel therapies directed to LSC eradication via interference with their anchoring to the stem cell niche have encouraged on-going or future advanced phase III clinical trials. In the coming years, we may see a shift in the focus of AML treatment to LSC-directed therapies if the prospect of improved cure rates holds true. In the future, AML treatment should lean toward personalized therapies using combinations of these compounds plus mutation-targeted agents and/or targeted delivery of chemotherapy, aiming at LSC eradication with reduced side effects.

Location First: Targeting Acute Myeloid Leukemia Within Its Niche

Despite extensive research and development of new treatments, acute myeloid leukemia (AML)-backbone therapy has remained essentially unchanged over the last decades and is frequently associated with poor outcomes. Eradicating the leukemic stem cells (LSCs) is the ultimate challenge in the treatment of AML. Emerging evidence suggests that AML remodels the bone marrow (BM) niche into a leukemia-permissive microenvironment while suppressing normal hematopoiesis. The mechanism of stromal-mediated protection of leukemic cells in the BM is complex and involves many adhesion molecules, chemokines, and cytokines. Targeting these factors may represent a valuable approach to complement existing therapies and overcome microenvironment-mediated drug resistance. Some strategies for dislodging LSCs and leukemic blasts from their protective niche have already been tested in patients and are in different phases of the process of clinical development. Other strategies, such as targeting the stromal cells remodeling processes, remain at pre-clinical stages. Development of humanized xenograft mouse models, which overcome the mismatch between human leukemia cells and the mouse BM niche, is required to generate physiologically relevant, patient-specific human niches in mice that can be used to unravel the role of human AML microenvironment and to carry out preclinical studies for the development of new targeted therapies.

An Auristatin nanoconjugate targeting CXCR4+ leukemic cells blocks acute myeloid leukemia dissemination

Background

Current acute myeloid leukemia (AML) therapy fails to eliminate quiescent leukemic blasts in the bone marrow, leading to about 50% of patient relapse by increasing AML burden in the bone marrow, blood, and extramedullar sites. We developed a protein-based nanoparticle conjugated to the potent antimitotic agent Auristatin E that selectively targets AML blasts because of their CXCR4 receptor overexpression (CXCR4+) as compared to normal cells. The therapeutic rationale is based on the involvement of CXCR4 overexpression in leukemic blast homing and quiescence in the bone marrow, and the association of these leukemic stem cells with minimal residual disease, dissemination, chemotherapy resistance, and lower patient survival.

Methods

Monomethyl Auristatin E (MMAE) was conjugated with the CXCR4 targeted protein nanoparticle T22-GFP-H6 produced in E. coli. Nanoconjugate internalization and in vitro cell viability assays were performed in CXCR4+ AML cell lines to analyze the specific antineoplastic activity through the CXCR4 receptor. In addition, a disseminated AML animal model was used to evaluate the anticancer effect of T22-GFP-H6-Auristatin in immunosuppressed NSG mice (n = 10/group). U of Mann-Whitney test was used to consider if differences were significant between groups.

Results

T22-GFP-H6-Auristatin was capable to internalize and exert antineoplastic effects through the CXCR4 receptor in THP-1 and SKM-1 CXCR4+ AML cell lines. In addition, repeated administration of the T22-GFP-H6-Auristatin nanoconjugate (9 doses daily) achieves a potent antineoplastic activity by internalizing specifically in the leukemic cells (luminescent THP-1) to selectively eliminate them. This leads to reduced involvement of leukemic cells in the bone marrow, peripheral blood, liver, and spleen, while avoiding toxicity in normal tissues in a luminescent disseminated AML mouse model.

Conclusions

A novel nanoconjugate for targeted drug delivery of Auristatin reduces significantly the acute myeloid leukemic cell burden in the bone marrow and blood and blocks its dissemination to extramedullar organs in a CXCR4+ AML model. This selective drug delivery approach validates CXCR4+ AML cells as a target for clinical therapy, not only promising to improve the control of leukemic dissemination but also dramatically reducing the severe toxicity of classical AML therapy.

Significance of CXCL12/CXCR4 Ligand/Receptor Axis in Various Aspects of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is defined as an aggressive disorder which is described by accumulation of immature malignant cells into the bone marrow. Chemokine-receptor axes are defined as factors involved in AML pathogenesis and prognosis. The chemokine receptor CXCR4 along with its ligand, CXCL12 fit in important players that are actively involved in the cross-talk between leukemia cells and bone marrow microenvironment. Therefore, according to the above introductory comments, in this review article, we have focused on delineating some parts played by CXCL12/CXCR4 axis in various aspects of AML malignancy. Targeting both leukemic and stromal cell interaction is nowadays accepted as a wide and attractive strategy for improving the outcome of treatment in AML in a non-cell autonomous manner. This strategy might be employed in a wide variety of AML patients regardless of their causative mutations. In addition to several potential targets involved in the disruption of malignant leukemic cells from their specific protective niches, compounds which interfere with CXCL12/CXCR4 axis have also been explored in multiple early-phase established clinical trials. Moreover, extensive research programs are exploring novel leading mechanisms for leukemia-stromal interactions that appear to find out novel therapeutic targets within the near future.

Dissecting the role of the CXCL12/CXCR4 axis in acute myeloid leukaemia

Acute myeloid leukaemia (AML) is the most common adult acute leukaemia with the lowest survival rate. It is characterised by a build-up of immature myeloid cells anchored in the protective niche of the bone marrow (BM) microenvironment. The CXCL12/CXCR4 axis is central to the pathogenesis of AML as it has fundamental control over AML cell adhesion into the protective BM niche, adaptation to the hypoxic environment, cellular migration and survival. High levels of CXCR4 expression are associated with poor relapse-free and overall survival. The CXCR4 ligand, CXCL12 (SDF-1), is expressed by multiple cells types in the BM, facilitating the adhesion and survival of the malignant clone. Blocking the CXCL12/CXCR4 axis is an attractive therapeutic strategy providing a 'multi-hit' therapy that both prevents essential survival signals and releases the AML cells from the BM into the circulation. Once out of the protective niche of the BM they would be more susceptible to destruction by conventional chemotherapeutic drugs. In this review, we disentangle the diverse roles of the CXCL12/CXCR4 axis in AML. We then describe multiple CXCR4 inhibitors, including small molecules, peptides, or monoclonal antibodies, which have been developed to date and their progress in pre-clinical and clinical trials. Finally, the review leads us to the conclusion that there is a need for further investigation into the development of a 'multi-hit' therapy that targets several signalling pathways related to AML cell adhesion and maintenance in the BM.

In vitro and in vivo efficacy of an anti-CD203c conjugated antibody (AGS-16C3F) in mouse models of advanced systemic mastocytosis

Antibody-drug conjugates (ADCs) are a new class of therapeutics that use antibodies to deliver potent cytotoxic drugs selectively to cancer cells. CD203c, an ecto-nucleotide pyrophosphatase-phosphodiesterase 3, is overexpressed on neoplastic mast cells (MCs) in systemic mastocytosis (SM), thus representing a promising target for antibody-mediated therapy. In this study, we have found that human neoplastic MC lines (ROSA^KIT D816V and ROSA^{KIT D816V-Gluc}), which express high levels of CD203c, are highly and specifically sensitive to the antiproliferative effects of an ADC against CD203c (AGS-16C3F). In these cell lines, AGS-16C3F induced cell apoptosis at very low concentrations. To characterize the effects of AGS-16C3F on leukemia progression in vivo, ROSA^{KIT D816V-Gluc} NOD-SCID γ mouse models of advanced SM (AdvSM) were treated with AGS-16C3F or an ADC control for 2 weeks. Whereas AGS-16C3F had no apparent toxicity in xenotransplanted mice, in vivo neoplastic MC burden significantly decreased in both hematopoietic and nonhematopoietic organs. Furthermore, animals treated with AGS-16C3F had prolonged survival compared with the animals treated with control ADC, and AGS-16C3F efficiently prevented disease relapse. In conclusion, these preclinical studies identified CD203c as a novel therapeutic target on neoplastic MCs, and AGS-16C3F as a promising ADC for the treatment of patients with AdvSM.

Role of CXCL12 and CXCR4 in the pathogenesis of hematological malignancies

The chemokine receptor CXCR4 and its ligand stromal cell-derived factor-1 (SDF-1/CXCL12) are important players in the cross-talk among lymphoma, myeloma and leukemia cells and their microenvironments. In hematological malignancies and solid tumors, the overexpression of CXCR4 on the cell surface has been shown to be responsible for disease progression, increasing tumor cell survival and chemoresistance and metastasis to organs with high CXCL12 levels (e.g., lymph nodes and bone marrow (BM)). Furthermore, the overexpression of CXCR4 has been found to have prognostic significance for disease progression in many type of tumors including lymphoma, leukemia, glioma, and prostate, breast, colorectal, renal, and hepatocellular carcinomas. In leukemia, CXCR4 expression granted leukemic blasts a higher capacity to seed into BM niches, thereby protecting leukemic cells from chemotherapy-induced apoptosis, and was correlated with shorter disease-free survival. In contrast, neutralizing the interaction of CXCL12/CXCR4 with a variety of antagonists induced apoptosis and differentiation and increased the chemosensitivity of lymphoma, myeloma, and leukemia cells. The role of CXCL12 and CXCR4 in the pathogenesis of hematological malignancies and the clinical therapeutic potential of CXCR4 antagonists in these diseases is discussed.

Redistribution, homing and organ-invasion of neoplastic stem cells in myeloid neoplasms

The development of a myeloid neoplasm is a step-wise process that originates from leukemic stem cells (LSC) and includes pre-leukemic stages, overt leukemia and a drug-resistant terminal phase. Organ-invasion may occur in any stage, but is usually associated with advanced disease and a poor prognosis. Sometimes, extra-medullary organ invasion shows a metastasis-like or even sarcoma-like destructive growth of neoplastic cells in local tissue sites. Examples are myeloid sarcoma, mast cell sarcoma and localized blast phase of chronic myeloid leukemia. So far, little is known about mechanisms underlying re-distribution and extramedullary dissemination of LSC in myeloid neoplasms. In this article, we discuss mechanisms through which LSC can mobilize out of the bone marrow niche, can transmigrate from the blood stream into extramedullary organs, can invade local tissue sites and can potentially create or support the formation of local stem cell niches. In addition, we discuss strategies to interfere with LSC expansion and organ invasion by targeted drug therapies.

Combination of PET and CXCR4-Targeted Peptide Molecule Agents for Noninvasive Tumor Monitoring

Precision medicine is emphasizing not only at individual but also at disease molecule level in modern medicine. Therefore, target-specific molecular agents are crucial for precise diagnosis and treatment. We developed a peptide agent that binds a critical chemokine receptor-CXCR4 and could be used to detect tumor status. Confocal images showed binding of the peptide agent to human osteosarcoma cells. Clinical gold-standard molecular imaging agent PET showed tumors had high glucose metabolism, CT showed that these xenograft tumors were calcified and displayed hypervascularity. Peptide imaging demonstrated that these tumors were CXCR4 positive. However, Western blot protein analysis revealed a discordance between the tumor and the CXCR4 targeted agent, suggesting that small changes in peptide sequences have profound effect on binding to their targets. We also demonstrated the molecular screening by modifying the peptide sequence and thereby altering the binding properties of the agent. In conclusion, this study demonstrates that small molecule peptide agents can be used as an additional diagnostic tool for precision medicine.

Use of polymeric CXCR4 inhibitors as siRNA delivery vehicles for the treatment of acute myeloid leukemia

Acute myeloid leukemia (AML) is the most common type of acute leukemia in adults and is associated with poor long-term survival often owing to relapse. Current treatments for AML are associated with considerable toxicity and are frequently not effective after relapse. Thus, it is important to develop novel therapeutic strategies. Short interfering RNA (siRNA)-based therapeutics targeting key oncogenes have been proposed as treatments for AML. We recently developed novel siRNA delivery polycations (PCX) based on AMD3100 (plerixafor), an FDA-approved inhibitor of the CXC chemokine receptor 4 (CXCR4). Inhibitors of CXCR4 have been shown to sensitize leukemia cells to chemotherapy. Therefore, PCX has the potential to target leukemia cells via two mechanisms: inhibition of CXCR4 and delivery of siRNAs against critical genes. In this report, we show that PCX exerts a cytotoxic effect on leukemia cells more effectively than other CXCR4 inhibitors, including AMD3100. In addition, we show that PCX can deliver siRNAs against the transcription factor RUNX1 to mouse and human leukemia cells. Overall, our study provides the first evidence that dual-function PCX/siRNA nanoparticles can simultaneously inhibit CXCR4 and deliver siRNAs, targeting key oncogenes in leukemia cells and that PCX/siRNA has clinical potential for the treatment of AML.

Osteogenic niche in the regulation of normal hematopoiesis and leukemogenesis

The bone marrow microenvironment, also known as the bone marrow niche, is a complex network of cell types and acellular factors that supports normal hematopoiesis. For many years, leukemia was believed to be caused by a series of genetic hits to hematopoietic stem and progenitor cells, which transform them to preleukemic, and eventually to leukemic, cells. Recent discoveries suggest that genetic alterations in bone marrow niche cells, particularly in osteogenic cells, may also cause myeloid leukemia in mouse models. The osteogenic niche, which consists of osteoprogenitors, preosteoblasts, mature osteoblasts, osteocytes and osteoclasts, has been shown to play a critical role in the maintenance and expansion of hematopoietic stem and progenitor cells as well as in their oncogenic transformation into leukemia stem/initiating cells. We have recently shown that acute myeloid leukemia cells induce osteogenic differentiation in mesenchymal stromal cells to gain a growth advantage. In this review, we discuss the role of the osteogenic niche in the maintenance of hematopoietic stem and progenitor cells, as well as in their transformation into leukemia cells. We also discuss the signaling pathways that regulate osteogenic niche-hematopoietic stem and progenitor cells or osteogenic niche-leukemic stem/initiating cell interactions in the bone marrow, together with novel approaches for therapeutically targeting these interactions.

Initial Report of a Phase I Study of LY2510924, Idarubicin, and Cytarabine in Relapsed/Refractory Acute Myeloid Leukemia

Background: The CXCR4/SDF-1α axis plays a vital role in the retention of stem cells within the bone marrow and downstream activation of cell survival signaling pathways. LY2510924, a second generation CXCR4, showed significant anti-leukemia activity in a murine AML model.

Methods: We conducted a phase I study to determine the safety and toxicity of LY2510924, idarubicin and cytarabine (IA) combination therapy in relapsed/refractory (R/R) AML. Eligible patients were 18–70 years of age receiving up to salvage 3 therapy. A peripheral blood absolute blast count of < 20,000/μL was required for inclusion. LY2510924 was administered daily for 7 days followed by IA from day 8. Two dose escalation levels (10 and 20 mg) were evaluated, with a plan to enroll up to 12 patients in the phase I portion.

Results: The median age of the enrolled patients (n = 11) was 55 years (range, 19–70). Median number of prior therapies was 1 (1–3). Six and five patients were treated at dose-levels “0” (10 mg) and “1” (20 mg), respectively. Only one patient experiencing a dose limiting toxicity (grade 3 rash and myelosuppression). Three and one complete responses were observed at dose-levels “0” and “1,” respectively; the overall response rate (ORR) was 36% (4 of 11 patients). A ≥ 50% decrease in CXCR4 mean fluorescence intensity was observed in 4 of 9 patients by flow cytometry, indicating incomplete suppression of CXCR4-receptor occupancy.

Conclusions: The combination of LY2510924 with IA is safe in R/R AML. Dose-escalation to a 30 mg LY2510924 dose is planned to achieve complete blockade of CXCR4 receptor occupancy, followed by expansion phase at the recommended phase 2 dose-level.

Role of the microenvironment in myeloid malignancies

The bone marrow microenvironment (BMM) regulates the fate of hematopoietic stem cells (HSCs) in homeostatic and pathologic conditions. In myeloid malignancies, new insights into the role of the BMM and its cellular and molecular actors in the progression of the diseases have started to emerge. In this review, we will focus on describing the major players of the HSC niche and the role of the altered niche function in myeloid malignancies, more specifically focusing on the mesenchymal stroma cell compartment.

Targeting primary acute myeloid leukemia with a new CXCR4 antagonist IgG1 antibody (PF-06747143)

The chemokine receptor CXCR4 mediates cell anchorage in the bone marrow (BM) microenvironment and is overexpressed in 25–30% of patients with acute myeloid leukemia (AML). Here we have shown that a new CXCR4 receptor antagonist IgG1 antibody (PF-06747143) binds strongly to AML cell lines and to AML primary cells inhibiting their chemotaxis in response to CXCL12. PF-06747143 also induced cytotoxicity in AML cells via Fc-effector function. To characterize the effects of PF-06747143 on leukemia progression, we used two different patient-derived xenograft (PDX) models: Patient 17^CXCR4-low and P15^CXCR4-high models, characterized by relatively low and high CXCR4 expression, respectively. Weekly administration of PF-06747143 to leukemic mice significantly reduced leukemia development in both models. Secondary transplantation of BM cells from PF-06747143-treated or IgG1 control-treated animals showed that leukemic progenitors were also targeted by PF-06747143. Administration of a single dose of PF-06747143 to PDX models induced rapid malignant cell mobilization into the peripheral blood (PB). These findings support evaluation of this antibody in AML therapy, with particular appeal to patients resistant to chemotherapy and to unfit patients, unable to tolerate intensive chemotherapy.

Engraftment of chronic myelomonocytic leukemia cells in immunocompromised mice supports disease dependency on cytokines

Chronic myelomonocytic leukemia (CMML) is a clonal hematopoietic stem cell disorder that typically associates with mutations in epigenetic, splicing, and signaling genes. Genetically modified mouse models only partially recapitulate the disease phenotype, whereas xenotransplantation of CMML cells in immunocompromised mice has been rarely successful so far. Here, CMML CD34⁺ cells sorted from patient bone marrow (BM) or peripheral blood (PB) were injected intravenously into NSG (NOD/LtSz-scid IL2rγnull) mice and NSG mice engineered to express human granulo-monocyte colony-stimulating factor, stem cell factor, and interleukin-3 (NSGS mice). Fifteen out of 16 patient samples (94%) successfully engrafted into NSG or NSGS or both mouse strains. The expansion of human cells, predominant in the BM, was also observed in the spleen and the PB and was greatly enhanced in mice producing the 3 human cytokines. Gene mutations identified in engrafted cells were mostly similar to those identified in patient cells before injection. Successful secondary engraftment was obtained in NSGS mice in 3 out of 10 attempts. Thus, primary CMML leukemic cells expand much better in NSGS compared with NSG mice with limited efficacy of secondary transplant.

Inhibition of CXCL12/CXCR4 axis as a potential targeted therapy of advanced gastric carcinoma

The whole outcome for patients with gastric carcinoma (GC) is very poor because most of them remain metastatic disease during survival even at diagnosis or after surgery. Despite many improvements in multiple strategies of chemotherapy, immunotherapy, and targeted therapy, exploration of novel alternative therapeutic targets is still warranted. Chemokine receptor 4 (CXCR4) and its chemokine ligand 12 (CXCL12) have been identified with significantly elevated levels in various malignancies including GC, which correlates with the survival, proliferation, angiogenesis, and metastasis of tumor cells. Increasing experimental evidence suggests an implication of inhibition of CXCL12/CXCR4 axis as a promising targeted therapy, although there are rare trials focused on the therapeutic efficacy of CXCR4 inhibitors in GC until recently. Therefore, it is reasonable to infer that specific antagonists or antibodies targeting CXCL12/CXCR4 axis alone or combined with chemotherapy will be effective and worthy of further translational studies as a potential treatment strategy in advanced GC.

Targeting the CXCL12/CXCR4 axis in acute myeloid leukemia: from bench to bedside

The interactions between the cancerous cells of acute myeloid leukemia (AML) and the bone marrow (BM) microenvironment have been postulated to be important for resistance to chemotherapy and disease relapse in AML. The chemokine receptor CXC chemokine receptor 4 (CXCR4) and its ligand, CXC motif ligand 12 (CXCL12), also known as stromal cell-derived factor 1α, are key mediators of this interaction. CXCL12 is produced by the BM microenvironment, binds and activates its cognate receptor CXCR4 on leukemic cells, facilitates leukemia cell trafficking and homing in the BM microenvironment, and keeps leukemic cells in close contact with the stromal cells and extracellular matrix that constitutively generate growth-promoting and anti-apoptotic signals. Indeed, a high level of CXCR4 expression on AML blasts is known to be associated with poor prognosis. Recent preclinical and clinical studies have revealed the safety and potential clinical utility of targeting the CXCL12/CXCR4 axis in AML with different classes of drugs, including small molecules, peptides, and monoclonal antibodies. In this review, we describe recent evidence of targeting these leukemia-stroma interactions, focusing on the CXCL12/CXCR4 axis. Related early phase clinical studies will be also introduced.

Genetic hierarchy and temporal variegation in the clonal history of acute myeloid leukaemia

In acute myeloid leukaemia (AML) initiating pre-leukaemic lesions can be identified through three major hallmarks: their early occurrence in the clone, their persistence at relapse and their ability to initiate multilineage haematopoietic repopulation and leukaemia in vivo. Here we analyse the clonal composition of a series of AML through these characteristics. We find that not only DNMT3A mutations, but also TET2, ASXL1 mutations, core-binding factor and MLL translocations, as well as del(20q) mostly fulfil these criteria. When not eradicated by AML treatments, pre-leukaemic cells with these lesions can re-initiate the leukaemic process at various stages until relapse, with a time-dependent increase in clonal variegation. Based on the nature, order and association of lesions, we delineate recurrent genetic hierarchies of AML. Our data indicate that first lesions, variegation and treatment selection pressure govern the expansion and adaptive behaviour of the malignant clone, shaping AML in a time-dependent manner.

Pre-leukaemic clones, together with the propensity to cause disease in mice, are characterized by appearing early in myeloid leukaemia and being found at relapse. Here, the authors identify clones in human samples and find that they are characterized by hierarchically organized genetic lesions, which can be used to track evolution of the disease.

CXCL12/CXCR4 pathway is activated by oncogenic JAK2 in a PI3K-dependent manner

JAK2 activation is the driver mechanism in BCR-ABL-negative myeloproliferative neoplasms (MPN). These diseases are characterized by an abnormal retention of hematopoietic stem cells within the bone marrow microenvironment and their increased trafficking to extramedullary sites. The CXCL12/CXCR4 axis plays a central role in hematopoietic stem cell/ progenitor trafficking and retention in hematopoietic sites. The present study explores the crosstalk between JAK2 and CXCL12/CXCR4 signaling pathways in MPN. We show that JAK2, activated by either MPL-W515L expression or cytokine stimulation, cooperates with CXCL12/CXCR4 signaling to increase the chemotactic response of human cell lines and primary CD34⁺ cells through an increased phosphatidylinositol-3-kinase (PI3K) signaling. Accordingly, primary myelofibrosis (MF) patient cells demonstrate an increased CXCL12-induced chemotaxis when compared to controls. JAK2 inhibition by knock down or chemical inhibitors decreases this effect in MPL-W515L expressing cell lines and reduces the CXCL12/CXCR4 signaling in some patient primary cells. Taken together, these data indicate that CXCL12/CXCR4 pathway is overactivated in MF patients by oncogenic JAK2 that maintains high PI3K signaling over the threshold required for CXCR4 activation. These results suggest that inhibition of this crosstalk may contribute to the therapeutic effects of JAK2 inhibitors.

Mesenchymal Stem and Progenitor Cells in Normal and Dysplastic Hematopoiesis-Masters of Survival and Clonality?

Myelodysplastic syndromes (MDS) are malignant hematopoietic stem cell disorders that have the capacity to progress to acute myeloid leukemia (AML). Accumulating evidence suggests that the altered bone marrow (BM) microenvironment in general, and in particular the components of the stem cell niche, including mesenchymal stem cells (MSCs) and their progeny, play a pivotal role in the evolution and propagation of MDS. We here present an overview of the role of MSCs in the pathogenesis of MDS, with emphasis on cellular interactions in the BM microenvironment and related stem cell niche concepts. MSCs have potent immunomodulatory capacities and communicate with diverse immune cells, but also interact with various other cellular components of the microenvironment as well as with normal and leukemic stem and progenitor cells. Moreover, compared to normal MSCs, MSCs in MDS and AML often exhibit altered gene expression profiles, an aberrant phenotype, and abnormal functional properties. These alterations supposedly contribute to the “reprogramming” of the stem cell niche into a disease-permissive microenvironment where an altered immune system, abnormal stem cell niche interactions, and an impaired growth control lead to disease progression. The current article also reviews molecular targets that play a role in such cellular interactions and possibilities to interfere with abnormal stem cell niche interactions by using specific targeted drugs.

Inhibition of CXCR4 by LY2624587, a Fully Humanized Anti-CXCR4 Antibody Induces Apoptosis of Hematologic Malignancies

SDF-1 and CXCR4 are a chemokine and chemokine receptor pair playing critical roles in tumorigenesis. Overexpression of CXCR4 is a hallmark of many hematological malignancies including acute myeloid leukemia, chronic lymphocytic leukemia and non-Hodgkin’s lymphoma, and generally correlates with a poor prognosis. In this study, we developed a humanized anti-CXCR4 monoclonal antibody, LY2624587 as a potent CXCR4 antagonist that was advanced into clinical study for cancer. LY2624587 blocked SDF-1 binding to CXCR4 with an IC₅₀ of 0.26 nM, and inhibited SDF-1-induced GTP binding with a K_b of 0.66 nM. In human lymphoma U937 and leukemia CCRF-CEM cells expressing endogenous CXCR4, LY2624587 inhibited SDF-1-induced cell migration with IC₅₀ values of 3.7 and 0.26 nM, respectively. This antibody also inhibited CXCR4 and SDF-1 mediated cell signaling including activation of MAPK and AKT in tumor cells expressing CXCR4. Bifocal microscopic and flow cytometry analyses revealed that LY2624587 mediated receptor internalization and caused CXCR4 down-regulation on the cell surface. In human hematologic cancer cells, LY2624587 caused dose dependent apoptosis in vitro and in vivo. In mouse xenograft models developed with human leukemia and lymphoma cells expressing high levels of CXCR4, LY2624587 exhibited dose-dependent tumor growth inhibition and provided significant survival benefit in a disseminated lymphoma model. Collectively, we have demonstrated that CXCR4 inhibition by LY2624587 has the potential for the treatment of human hematological malignancies.

Are there any new insights for G-CSF and/or AMD3100 in chemotherapy of haematological malignants?

AML is a common life-threatening blood system malignancy. The treatment of AML continues to face greater challenges. An abnormal haematopoietic niche with high adhesion and proliferation might be the root cause of resistance and relapse. Most leukaemia cells are stored in the endosteal niche and recess in the G0 phase, and they are not sensitive to varieties of radiotherapies and chemotherapies. G-CSF and AMD3100 are increasingly used in priming chemotherapy. G-CSF can promote leukaemia cells to the cell cycle, which improves the complete remission rate of leukaemia patients. AMD3100, the novel CXCR4 antagonist, could also potentially promote leukaemia cells to cell cycle and improve the susceptibility of leukaemia cells to chemotherapeutic agents. The combination of them enhances anti-leukaemia effect. So in this review, we explore the function of G-CSF and/or AMD3100 in the priming chemotherapy of haematological malignants.

A CXCR4 antagonist leads to tumor suppression by activation of immune cells in a leukemia-induced microenvironment

The bone marrow microenvironment (BMM) provides a protective niche that supports growth and survival of normal and leukemic hematopoietic stem cells. The SDF-1/CXCR4 interaction is critical for regulation of homing to and retention of hematopoietic cells in the bone marrow (BM), which leads to increased chemoresistance. SDF-1/CXCR4 plays pivotal roles in cross-interactions between blasts and the BMM to prevent retention and mobilization of leukemic cells, as well as in normal hematopoiesis including the development of immune cells. We show that the CXCR4 antagonist, plerixafor, decreased the level of CXCR4 expression and inhibited SDF-1-induced migration of leukemic cells. Further, the inhibition of the interaction between leukemic cells and the BMM by the plerixafor enhanced cytotoxic activity of immune cells as a result of increased susceptibility of leukemic cells to chemotherapeutic agents such as cytosine arabinoside (Ara-C) in a mouse model of acute myeloid leukemia (AML), suggesting biological effects of the BMM through immune cell activation. Because alterations in the BMM promote retention and survival of leukemic cells, targeting the niche is regarded as an advanced strategy to eradicate drug-resistant leukemic blasts. This study demonstrates that the effects of CXCR4 inhibition on blast suppression and immune cell function in the tumor microenvironment and chemotherapy with plerixafor represents an advanced therapeutic strategy of targeting the leukemic niche.

Differential hypoxic regulation of the microRNA-146a/CXCR4 pathway in normal and leukemic monocytic cells: impact on response to chemotherapy

High expression of the chemokine receptor 4, CXCR4, associated with a negative prognosis in acute myeloid leukemia, is related to hypoxia. Because CXCR4 expression is under the post-transcriptional control of microRNA-146a in normal and leukemic monocytic cells, we first investigated the impact of hypoxia on microRNA-146a and CXCR4 expression during monocytopoiesis and in acute monocytic leukemia. We then analyzed the effects of hypoxia on drug sensitivity of CXCR4-expressing leukemic cells. We found that microRNA-146a is a target of hypoxia-inducible factor-1α or -2α in relation to the stage of monocytopoiesis and the level of hypoxia, and demonstrated the regulation of the microRNA-146a/CXCR4 pathway by hypoxia in monocytes derived from CD34(+) cells. Thus, in myeloid leukemic cell lines, hypoxia-mediated control of the microRNA-146a/CXCR4 pathway depends only on the capacity of hypoxia-inducible factor-1α to up-regulate microRNA-146a, which in turn decreases CXCR4 expression. However, at variance with normal monocytic cells and leukemic cell lines, in acute monocytic leukemia overexpressing CXCR4, hypoxia up-modulates microRNA-146a but fails to down-modulate CXCR4 expression. We then investigated the effect of hypoxia on the response of leukemic cells to chemotherapy alone or in combination with stromal-derived factor-1α. We found that hypoxia increases stromal-derived factor-1α-induced survival of leukemic cells by decreasing their sensitivity to anti-leukemic drugs. Altogether, our results demonstrate that hypoxia-mediated regulation of microRNA-146a, which controls CXCR4 expression in monocytic cells, is lost in acute monocytic leukemia, thus contributing to maintaining CXCR4 overexpression and protecting the cells from anti-leukemic drugs in the hypoxic bone marrow microenvironment.

Antileukemia activity of the novel peptidic CXCR4 antagonist LY2510924 as monotherapy and in combination with chemotherapy

Targeting the stromal cell-derived factor 1α (SDF-1α)/C-X-C chemokine receptor type 4 (CXCR4) axis has been shown to be a promising therapeutic approach to overcome chemoresistance in acute myeloid leukemia (AML). We investigated the antileukemia efficacy of a novel peptidic CXCR4 antagonist, LY2510924, in preclinical models of AML. LY2510924 rapidly and durably blocked surface CXCR4 and inhibited stromal cell-derived factor 1 (SDF-1)α-induced chemotaxis and prosurvival signals of AML cells at nanomolar concentrations more effectively than the small-molecule CXCR4 antagonist AMD3100. In vitro, LY2510924 chiefly inhibited the proliferation of AML cells with little induction of cell death and reduced protection against chemotherapy by stromal cells. In mice with established AML, LY2510924 caused initial mobilization of leukemic cells into the circulation followed by reduction in total tumor burden. LY2510924 had antileukemia effects as monotherapy as well as in combination with chemotherapy. Gene expression profiling of AML cells isolated from LY2510924-treated mice demonstrated changes consistent with loss of SDF-1α/CXCR4 signaling and suggested reduced proliferation and induction of differentiation, which was proved by showing the attenuation of multiple prosurvival pathways such as PI3K/AKT, MAPK, and β-catenin and myeloid differentiation in vivo. Effective disruption of the SDF-1α/CXCR4 axis by LY2510924 may translate into effective antileukemia therapy in future clinical applications.

[Effects of SDF-1/CXCR4 signal pathway blockade by AMD3100 on the adhesion of leukemia cells to osteoblast niche and the drug resistance of leukemia cells]

OBJECTIVE

To study the blocking effect of CXCR4 inhibitor AMD3100 on the adhesion of leukemia cells to osteoblast niche, and the reversal of multidrug resistance in leukemia cells.

METHODS

Mesenchymal stem cells (MSCs) from leukemia patients were planted on the bio-derived bone scaffolds and then induced into osteoblasts to establish the bio-osteoblast niche. The levels of SDF-1were tested with ELISA. The leukemia cell line MV4-11 cells with FLT3-ITD mutation were inoculated into the bio-osteoblast niche to build a three-dimensional co- culture system. The expression level of CXCR4, adhesion and apoptosis rates of leukemia cells were observed by flow cytometry after incubation with AMD3100 and Ara-C for 24 h and 48 h.

RESULTS

(1)The supernatant levels of SDF-1 in cultured osteoblast were (304 ± 18), (410 ± 28) and (396 ± 16) pg/ml on 7 th, 14 th and 21 th day, respectively. It reached the highest on 14 th day. The expression level of CXCR4 in cultured MV4-11 cells was (72 ± 16)%. (2)Adhesion rate of MV4-11 cells to osteoblast niche was (40.1 ± 8.1)% after AMD3100 treatment for 24 h, while that of control group was (65.6 ± 12.1)% (P<0.05). (3)The apoptosis rate of MV4-11 cells incubated with AMD3100 for 24 h was (5.6 ± 0.8)%, while that of control group was (2.5 ± 0.5)%. The apoptosis rates of AMD3100-induced MV4-11 cells were (10.0 ± 2.4)%, (17.8 ± 2.3)% and (25.1 ± 2.4)% after treatment with Ara-C at 0.02, 0.20, 2.00 mg/ml respectively and they were (6.7 ± 1.0)%, (10.3 ± 1.5)%, (16.2 ± 3.1)% respectively in AMD3100-noninduced control group, the difference was significant (P<0.05).

CONCLUSION

AMD3100 can block the interaction between osteoblasts niches and leukemia cells, and play an important role in the reversal of multidrug resistance in leukemia cells.

Cancer stem cells in haematological malignancies

At least several types of human haematological malignancies can now be seen as 'stem-cell diseases'. The best-studied in this context is acute myeloid leukaemia (AML). It has been shown that these diseases are driven by a pool of 'leukaemia stem cells (LSC)', which remain in the quiescent state, have the capacity to survive and self-renew, and are responsible for the recurrence of cancer after classical chemotherapy. It has been understood that LSC must be eliminated in order to cure patients suffering from haematological cancers. Recent advances in LSC research have allowed for description of LSC phenotype and identification of potential targets for anti-LSC therapies. This concise review summarises the current view on LSC biology and targeted approaches against LSC.

[CXCR4: a new therapeutic target of the leukaemic cell? Role of the SDF-1/CXCR4 axis in acute myeloid leukaemia]

CXCR4, receptor of the chemokine SDF-1 (stromal cell-derived factor 1) plays a major role in the normal hematopoiesis but also in the biology of the leukaemic cell. This receptor is expressed on the surface of blasts and is a key molecule in "the anchoring" of the leukaemic stem cell (LSC) within the bone marrow niche. The interactions of the LSC with the bone marrow microenvironment promote survival signals and drug resistance. Recent flow cytometry analyses reported that CXCR4 expression levels have a major prognostic impact in acute myeloid leukaemia (AML). CXCR4 expression is associated with poor prognosis and can be useful to stratify patients, according to their phenotype, in order to establish risk-adapted strategies. Newly diagnosed AML are now routinely stratified according to molecular markers which guide prognosis and treatment. Many leukaemia are composed of multiples subclones with differential susceptibility to treatment and specific targeted therapies are missing. Despite therapeutic improvements for the treatment of AML, long term survival remains poor. Targeting CXCR4 is a novel promising approach of therapy. CXCR4 antagonists are used in combination with chemotherapy in preclinical and clinical studies. This review summarises our current knowledge regarding the key role of CXCR4 in AML and discusses how targeting this pathway could provide an interesting approach to eradicate the LSC.

Recruitment of bone marrow derived cells during anti-angiogenic therapy in GBM: the potential of combination strategies

Glioblastoma (GBM) is a highly vascular tumor characterized by rapid and invasive tumor growth, followed by oxygen depletion, hypoxia and neovascularization, which generate a network of disorganized, tortuous and permeable vessels. Recruitment of bone marrow derived cells (BMDC) is crucial for vasculogenesis. These cells may act as vascular progenitors by integrating into the newly formed blood vessels or as vascular modulators by releasing pro-angiogenic factors. In patients with recurrent GBM, anti-vascular endothelial growth factor (VEGF) therapy has been evaluated in combination with chemotherapy, yielding improvements in progression-free survival (PFS). However, benefits are temporary as vascular tumors acquire angiogenic pathways independently of VEGF. Specifically, acute hypoxia following prolonged VEGF depletion induces the recruitment of certain myeloid cell subpopulations, which highly contribute to treatment refractoriness. Here we review the molecular mechanisms of neovascularization in relation to bevacizumab therapy with special emphasis on the recruitment of BMDCs and possible combination therapies for GBM patients.

Immunotherapeutic strategies for relapse control in acute myeloid leukemia

Despite that the initial phases of chemotherapy induce disappearance of leukemic cells in many patients with acute myeloid leukemia (AML), the prevention of life-threatening relapses in the post-remission phase remains a significant clinical challenge. Allogeneic bone marrow transplantation, which is available for a minority of patients, efficiently prevents recurrences of leukemia by inducing immune-mediated elimination of leukemic cells, and over the past decades, numerous immunotherapeutic protocols have been developed aiming to mimic the graft-versus-leukemia reaction for the prevention of relapse. Here we review past and present strategies for relapse control with focus on overcoming leukemia-related immunosuppression in AML. We envisage future treatment protocols, in which systemic immune activators, such as vaccines, dendritic cell-based therapies, engineered variants of IL-2, or IL-15, are combined with agents that counter immunosuppression mediated by, e.g., the PD/PDL interaction, CTLA-4, CD200, reactive oxygen species, IDO expression, CXCR4, or the KIR/class I interaction, based on characteristics of the prevailing malignant clone. This combinatorial approach may pave the way for individualized immunotherapy in AML.

Dynamic chemotherapy-induced upregulation of CXCR4 expression: a mechanism of therapeutic resistance in pediatric AML

Cure rates in pediatric acute leukemias remain suboptimal. Overexpression of the cell-surface chemokine receptor CXCR4 is associated with poor outcome in acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). Certain nonchemotherapeutic agents have been shown to modulate CXCR4 expression and alter leukemia interactions with stromal cells in the bone marrow microenvironment. Because chemotherapy is the mainstay of AML treatment, it was hypothesized that standard cytotoxic chemotherapeutic agents induce dynamic changes in leukemia surface CXCR4 expression, and that chemotherapy-induced upregulation of CXCR4 represents a mechanism of acquired therapeutic resistance. Here, it was shown that cell lines variably upregulate CXCR4 with chemotherapy treatment. Those that showed upregulation were differentially protected from chemotherapy-induced apoptosis when cocultured with stroma. The functional effects of chemotherapy-induced CXCR4 upregulation in an AML cell line (MOLM-14, which harbors consistent upregulated CXCR4) and clinical specimens were explored. Importantly, enhanced stromal-cell derived factor-1α (SDF1A/CXCL12)-mediated chemotaxis and stromal protection from additional chemotherapy-induced apoptosis was found. Furthermore, treatment with plerixafor, a CXCR4 inhibitor, preferentially decreased stromal protection with higher chemotherapy-induced upregulation of surface CXCR4. Thus, increased chemokine receptor CXCR4 expression after treatment with conventional chemotherapy may represent a mechanism of therapeutic resistance in pediatric AML.

IMPLICATIONS

CXCR4 may be a biomarker for the stratification and optimal treatment of patients using CXCR4 inhibitors.