Relation between CXCR-4 expression, Flt3 mutations, and unfavorable prognosis of adult acute myeloid leukemia

Understanding mechanisms of resistance to FLT3 inhibitors in adult FLT3-mutated acute myeloid leukemia to guide treatment strategy

The presence of FLT3 mutations, including the most common FLT3-ITD (internal tandem duplications) and FLT3-TKD (tyrosine kinase domain), is associated with an unfavorable prognosis in patients affected by acute myeloid leukemia (AML). In this setting, in recent years, new FLT3 inhibitors have demonstrated efficacy in improving survival and treatment response. Nevertheless, the development of primary and secondary mechanisms of resistance poses a significant obstacle to their efficacy. Understanding these mechanisms is crucial for developing novel therapeutic approaches to overcome resistance and improve the outcomes of patients. In this context, the use of novel FLT3 inhibitors and the combination of different targeted therapies have been studied. This review provides an update on the molecular alterations involved in the resistance to FLT3 inhibitors, and describes how the molecular monitoring may be used to guide treatment strategy in FLT3-mutated AML.

Study on the mechanism of CXCL12/CXCR4-axis-mediated upregulation of IL-8 and IL-6 on the biological function of acute T lymphocyte leukaemia cells

Blocking the CXCL12/CXCR4 axis can alter the biological functions of leukaemia cells. We hypothesise that interleukin (IL)-8 and IL-6 play an important role in this process. To test this hypothesis, we established a co-culture model of leukaemia cells and bone marrow stromal cells. Treatment of cells with AMD3100, a CXCR4 antagonist, and G-CSF blocked the CXCL12/CXCR4 axis, inducing biological changes in the leukaemia cells and altering IL-8 and IL-6 levels. Subsequently, after stimulating the CXCL12/CXCR4 axis, specific pathway blockers were employed to assess the role of four candidate signalling pathways in this process. ELISA results confirmed that MG-132 (10 μm) inhibits IL-8 expression and that the NF-κB signalling pathway contributes to this effect. Moreover, treatment with Perifosine, an AKT inhibitor, inhibited IL-6 expression. In addition, changes in the NF-κB signalling pathway inhibited IL-8 expression. Treatment with SP600125, a Jun N-terminal kinase inhibitor, and Perifosine also inhibited IL-8 expression; however, this effect occurred later. IL-6 expression was also lower in the Perifosine group; hence, inhibiting the PI3K/AKT signalling pathway can reduce IL-6 expression. This process requires the participation of multiple signalling pathways to regulate IL-8 and IL-6 expression. Therefore, the associated mechanism is likely to be highly intricate, with potential cross-effects that may impact leukaemia pathogenesis. IL-6 and IL-8 are physiologically regulated by the CXCL12/CXCR4 axis, while the NF-κB and JNK/AP-1 pathways are required for IL-8 expression in T-cell acute lymphoblastic leukaemia. Accordingly, by upregulating IL-8, the bone marrow microenvironment and CXCL12/CXCR4 axis may contribute to T-cell acute lymphoblastic leukaemia pathogenesis.

Relapse of acute myeloid leukemia after allogeneic stem cell transplantation: immune escape mechanisms and current implications for therapy

Acute myeloid leukemia (AML) is a heterogeneous disease characterized by the expansion of immature myeloid cells in the bone marrow (BM) and peripheral blood (PB) resulting in failure of normal hematopoiesis and life-threating cytopenia. Allogeneic hematopoietic stem cell transplantation (allo-HCT) is an established therapy with curative potential. Nevertheless, post-transplant relapse is common and associated with poor prognosis, representing the major cause of death after allo-HCT. The occurrence of relapse after initially successful allo-HCT indicates that the donor immune system is first able to control the leukemia, which at a later stage develops evasion strategies to escape from immune surveillance. In this review we first provide a comprehensive overview of current knowledge regarding immune escape in AML after allo-HCT, including dysregulated HLA, alterations in immune checkpoints and changes leading to an immunosuppressive tumor microenvironment. In the second part, we draw the line from bench to bedside and elucidate to what extend immune escape mechanisms of relapsed AML are yet exploited in treatment strategies. Finally, we give an outlook how new emerging technologies could help to improve the therapy for these patients, and elucidate potential new treatment options.

The interplay of FLT3 and CXCR4 in acute myeloid leukemia: an ongoing debate

FLT3 mutations are very frequent in AML and utilization of FLT3 inhibitors as approved treatment options are very common. Despite the initial success of inhibitor treatment, the development of resistances against this treatment is a major challenge in AML therapy. One of the mechanisms causing resistance is the homing of the leukemic cells in the protective niche of the bone marrow microenvironment (BMM). A pathway mediating homing to the BMM and leukemic cell survival is the CXCL12/CXCR4 axis. The analysis of patient samples in several independent studies indicated that FLT3-ITD expression led to higher CXCR4 surface expression. However, several in vitro studies reported contradictory findings, suggesting that FLT3-ITD signaling negatively influenced CXCR4 expression. In this commentary, we provide an overview summarizing the studies dealing with the relationship of FLT3 and CXCR4. Taken together, the current research status is not sufficient to answer the question whether FLT3 and CXCR4 act together or independently in leukemia progression. Systematic analyses in model cell systems are needed to understand the interplay between FLT3 and CXCR4, since this knowledge could lead to the development of more effective treatment strategies for AML patients.

Microenvironmental CXCL12 deletion enhances Flt3-ITD acute myeloid leukemia stem cell response to therapy by reducing p38 MAPK signaling

Fms-like tyrosine kinase 3 (Flt3) tyrosine kinase inhibitors (Flt3-TKI) have improved outcomes for patients with Flt3-mutated acute myeloid leukemia (AML) but are limited by resistance and relapse, indicating persistence of leukemia stem cells (LSC). Here utilizing a Flt3-internal tandem duplication (Flt3-ITD) and Tet2-deleted AML genetic mouse model we determined that FLT3-ITD AML LSC were enriched within the primitive ST-HSC population. FLT3-ITD LSC showed increased expression of the CXCL12 receptor CXCR4. CXCL12-abundant reticular (CAR) cells were increased in Flt3-ITD AML marrow. CXCL12 deletion from the microenvironment enhanced targeting of AML cells by Flt3-TKI plus chemotherapy treatment, including enhanced LSC targeting. Both treatment and CXCL12 deletion partially reduced p38 mitogen-activated protein kinase (p38) signaling in AML cells and further reduction was seen after treatment in CXCL12 deleted mice. p38 inhibition reduced CXCL12-dependent and -independent maintenance of both murine and human Flt3-ITD AML LSC by MSC and enhanced their sensitivity to treatment. p38 inhibition in combination with chemotherapy plus TKI treatment leads to greater depletion of Flt3-ITD AML LSC compared with CXCL12 deletion. Our studies support roles for CXCL12 and p38 signaling in microenvironmental protection of AML LSC and provide a rationale for inhibiting p38 signaling to enhance Flt3-ITD AML targeting.

Acute pancreatitis as initial presentation of acute myeloid leukemia-M2 subtype: A case report

BACKGROUND

Direct infiltration of the pancreas by acute myeloid leukemia (AML) with acute pancreatitis (AP) as an initial symptom is extremely rare. Only once in the literature, the leukemia cells in AML have been implicated as the cause of AP. Pancreatitis caused by a rare predisposing factor is often misdiagnosed as idiopathic pancreatitis or pancreatitis of other common causes. Severe AP (SAP) progresses rapidly with a high fatality rate. Therefore, it is important to identify the predisposing factors in the early stage of SAP, evaluate the condition, determine prognosis, formulate treatment plans, and prevent a recurrence. Here, we describe a case of SAP due to AML.

CASE SUMMARY

A 61-year-old man presented to the hospital with fever and persistent abdominal pain. Blood analysis presented significantly elevated serum amylase and severe thrombocytopenia. Computed tomography examination of the abdomen revealed peripancreatic inflammatory effusion. The patient had no common etiologies and risk factors for AP, but the concurrent severe thrombocytopenia could not be explained by pancreatitis. Finally, the bone marrow aspirate and biopsy inspection revealed the underlying reason for pancreatitis, AML (M2 type based on the French-American-British classifications system).

CONCLUSION

Direct infiltration of the pancrease by acute leukemia, particularly AML cells, is an infrequent cause of AP. Therefore, although AP is a rare extramedullary infiltration characteristic for AML patients, it should be considered when determining the etiology of AP.

T22-PE24-H6 Nanotoxin Selectively Kills CXCR4-High Expressing AML Patient Cells In Vitro and Potently Blocks Dissemination In Vivo

Despite advances in the development of targeted therapies for acute myeloid leukemia (AML), most patients relapse. For that reason, it is still necessary to develop novel therapies that improve treatment effectiveness and overcome drug resistance. We developed T22-PE24-H6, a protein nanoparticle that contains the exotoxin A from the bacterium Pseudomonas aeruginosa and is able to specifically deliver this cytotoxic domain to CXCR4+ leukemic cells. Next, we evaluated the selective delivery and antitumor activity of T22-PE24-H6 in CXCR4+ AML cell lines and BM samples from AML patients. Moreover, we assessed the in vivo antitumor effect of this nanotoxin in a disseminated mouse model generated from CXCR4+ AML cells. T22-PE24-H6 showed a potent, CXCR4-dependent antineoplastic effect in vitro in the MONO-MAC-6 AML cell line. In addition, mice treated with nanotoxins in daily doses reduced the dissemination of CXCR4+ AML cells compared to buffer-treated mice, as shown by the significant decrease in BLI signaling. Furthermore, we did not observe any sign of toxicity or changes in mouse body weight, biochemical parameters, or histopathology in normal tissues. Finally, T22-PE24-H6 exhibited a significant inhibition of cell viability in CXCR4high AML patient samples but showed no activity in CXCR4low samples. These data strongly support the use of T22-PE24-H6 therapy to benefit high-CXCR4-expressing AML patients.

The magnitude of CXCR4 signaling regulates resistance to quizartinib in FLT3/ITD+ cells via RUNX1

CXCR4 antagonists sensitize FLT3/ITD⁺ AML cells to FLT3 inhibitors; however, CXCR4 signaling can induce apoptosis in AML cells, raising the question of whether CXCR4 signaling exerts divergent effects on FLT3/ITD⁺ cells. The present study investigated the paradoxical function of CXCR4 in resistance to FLT3 inhibitors. The FLT3 inhibitor quizartinib significantly decreased the number of FLT3/ITD⁺ Ba/F3 cells, whereas 1 ng/ml CXCL12 showed a significant protective effect against quizartinib. In contrast, CXCL12 over 100 ng/ml significantly decreased FLT3/ITD⁺ cell viability with concomitant downregulation of Runx1. Moreover, the survival of FLT3/ITD⁺ Ba/F3 or MOLM13 cells with low surface CXCR4 expression incubated with quizartinib was significantly enhanced by 100 ng/ml CXCL12; however, this protective effect of CXCL12 against quizartinib was barely detected in cells with high surface CXCR4 expression. Although silencing Runx1 downregulated CXCR4 expression, RUNX1 expression levels were significantly higher in CXCR4^LOW FLT3/ITD⁺ Ba/F3 cells incubated with 100 ng/ml CXCL12 than in CXCR4^HIGH cells, coincident with an increase in FLT3 phosphorylation. Silencing RUNX1 partially abrogated resistance to quizartinib in CXCR4^LOW cells incubated with CXCL12, whereas ectopic RUNX1 significantly restored resistance in CXCR4^HIGH cells. These results indicate that CXCR4 signaling of different magnitudes paradoxically regulates resistance to quizartinib in FLT3/ITD⁺ cells via RUNX1.

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.

Phospho-Specific Flow Cytometry Reveals Signaling Heterogeneity in T-Cell Acute Lymphoblastic Leukemia Cell Lines

Several signaling pathways are aberrantly activated in T-ALL due to genetic alterations of their components and in response to external microenvironmental cues. To functionally characterize elements of the signaling network in T-ALL, here we analyzed ten signaling proteins that are frequently altered in T-ALL -namely Akt, Erk1/2, JNK, Lck, NF-κB p65, p38, STAT3, STAT5, ZAP70, Rb- in Jurkat, CEM and MOLT4 cell lines, using phospho-specific flow cytometry. Phosphorylation statuses of signaling proteins were measured in the basal condition or under modulation with H₂O₂, PMA, CXCL12 or IL7. Signaling profiles are characterized by a high variability across the analyzed T-ALL cell lines. Hierarchical clustering analysis documents that higher intrinsic phosphorylation of Erk1/2, Lck, ZAP70, and Akt, together with ZAP70 phosphorylation induced by H₂O₂, identifies Jurkat cells. In contrast, CEM are characterized by higher intrinsic phosphorylation of JNK and Rb and higher responsiveness of Akt to external stimuli. MOLT4 cells are characterized by higher basal STAT3 phosphorylation. These data document that phospho-specific flow cytometry reveals a high variability in intrinsic as well as modulated signaling networks across different T-ALL cell lines. Characterizing signaling network profiles across individual leukemia could provide the basis to identify molecular targets for personalized T-ALL therapy.

One-Chip Isolation of Drug-Resistant Acute Myeloid Leukemia Cells with CXCR4-Targeted Magnetic Fluorescent Nanoprobes

Drug resistance and relapse lead to high mortality in acute myeloid leukemia, and studies have shown that CXCR4 overexpression is highly correlated with poor prognosis and drug resistance in leukemia cells. Isolation and detection of AML cells with CXCR4 overexpression will be crucial to the treatment of AML. In this paper, magnetic nanoparticles were firstly prepared successfully by high-temperature thermal decomposition method, and then characterized by TEM, VSM and DLS. Subsequently CXCR4-targeted magnetic fluorescent nanoprobes conjugated with antibody 12G5 were constructed by stepwise coupling. In cell experiments, the obtained probes demonstrated excellent targeting efficacy to CXCR4 overexpressed AML cells HL-60. In addition, HL-60 cells labelled with the magnetic probes can be magnetic isolated successfully in one microfluidics chip, with efficiency of 82.92 ± 7.03%. Overall, this method utilizes the superiority of superparamagnetic nanomaterials and microfluidic technology to achieve the enrichment and capture of drug-resistant cells in a microfluidic chip, providing a new idea for the isolation and detective of drug-resistant acute myeloid leukemia cells.

Autophagy in acute myeloid leukemia: a paradoxical role in chemoresistance

Autophagy is a lysosomal degradation pathway that is constitutively active in almost every cell of our body at basal level. This self-eating process primarily serves to remove superfluous constituents of the cells and recycle the degraded products. Autophagy plays an essential role in cell homeostasis and can be enhanced in response to stressful conditions. Impairment in the regulation of the autophagic pathway is implicated in pathological conditions such as neurodegeneration, cardiac disorders, and cancer. However, the role of autophagy in cancer initiation and development is controversial and context-dependent. Evidence from various studies has shown that autophagy serves dual purpose and may assist in cancer progression or suppression. In the early stages of cancer initiation, autophagy acts as a quality control mechanism and prevents cancer development. When cancer is established and progresses to a later stage, autophagy helps in the survival of these cells through adaptation to stresses, including exposure to anti-cancer drugs. In this review, we highlight various studies on autophagic pathways and describe the role of autophagy in cancer, specifically acute myeloid leukemia (AML). We also discuss the prognostic significance of autophagy genes involved in AML leukemogenesis and implications in conferring resistance to chemotherapy.

CXCR4 Mediates Enhanced Cell Migration in CALM-AF10 Leukemia

Leukemia transformed by the CALM-AF10 chromosomal translocation is characterized by a high incidence of extramedullary disease, central nervous system (CNS) relapse, and a poor prognosis. Invasion of the extramedullary compartment and CNS requires leukemia cell migration out of the marrow and adherence to the cells of the local tissue. Cell adhesion and migration are increasingly recognized as contributors to leukemia development and therapeutic response. These processes are mediated by a variety of cytokines, chemokines, and their receptors, forming networks of both secreted and cell surface factors. The cytokines and cytokine receptors that play key roles in CALM-AF10 driven leukemia are unknown. We find high cell surface expression of the cytokine receptor CXCR4 on leukemia cells expressing the CALM-AF10 oncogenic protein, contributing to the migratory nature of this leukemia. Our discovery of altered cytokine receptor expression and function provides valuable insight into the propagation and persistence of CALM-AF10 driven leukemia.

Integrated RNAi screening identifies the NEDDylation pathway as a synergistic partner of azacytidine in acute myeloid leukemia

Treatment of acute myeloid leukemia (AML) remains challenging and novel targets and synergistic therapies still need to be discovered. We performed a high-throughput RNAi screen in three different AML cell lines and primary human leukemic blasts to identify genes that synergize with common antileukemic therapies. We used a pooled shRNA library that covered 5043 different genes and combined transfection with exposure to either azacytidine or cytarabine analog to the concept of synthetic lethality. Suppression of the chemokine CXCL12 ranked highly among the candidates of the cytarabine group. Azacytidine in combination with suppression of genes within the neddylation pathway led to synergistic results. NEDD8 and RBX1 inhibition by the small molecule inhibitor pevonedistat inhibited leukemia cell growth. These findings establish an in vitro synergism between NEDD8 inhibition and azacytidine in AML. Taken together, neddylation constitutes a suitable target pathway for azacytidine combination strategies.

A multivalent Ara-C-prodrug nanoconjugate achieves selective ablation of leukemic cells in an acute myeloid leukemia mouse model

Current therapy in acute myeloid leukemia (AML) is based on chemotherapeutic drugs administered at high doses, lacking targeting selectivity and displaying poor therapeutic index because of severe adverse effects. Here, we develop a novel nanoconjugate that combines a self-assembled, multivalent protein nanoparticle, targeting the CXCR4 receptor, with an Oligo-Ara-C prodrug, a pentameric form of Ara-C, to highly increase the delivered payload to target cells. This 13.4 nm T22-GFP-H6-Ara-C nanoconjugate selectively eliminates CXCR4⁺ AML cells, which are protected by its anchoring to the bone marrow (BM) niche, being involved in AML progression and chemotherapy resistance. This nanoconjugate shows CXCR4-dependent internalization and antineoplastic activity in CXCR4⁺ AML cells in vitro. Moreover, repeated T22-GFP-H6-Ara-C administration selectively eliminates CXCR4⁺ leukemic cells in BM, spleen and liver. The leukemic dissemination blockage induced by T22-GFP-H6-Ara-C is significantly more potent than buffer or Oligo-Ara-C-treated mice, showing no associated on-target or off-target toxicity and, therefore, reaching a highly therapeutic window. In conclusion, T22-GFP-H6-Ara-C exploits its 11 ligands-multivalency to enhance target selectivity, while the Oligo-Ara-C prodrug multimeric form increases 5-fold its payload. This feature combination offers an alternative nanomedicine with higher activity and greater tolerability than current intensive or non-intensive chemotherapy for AML patients.

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.

Clinical and prognostic relevance of CXCL12 expression in acute myeloid leukemia

Background

Accumulating studies have been made to understand the association between CXC chemokine ligand-12 (CXCL12)/CXC chemokine receptor 4 (CXCR4) and acute myeloid leukemia (AML). However, large-scale data analysis of potential relationship between CXCL12 and AML remains insufficient.

Methods

We collected abundant CXCL12 expression data and AML samples from several publicly available datasets. The CIBERSORT algorithm was used to quantify immune cell fractions and the online website of STRING was utilized for gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. The statistical analysis and graphical work were mainly performed via the R software.

Results

CXCL12 expression was extremely down-regulated in AML. Clinically, low CXCL12 expression was correlated with higher white blood cells (WBCs) (P < 0.0001), more blasts in bone marrow (BM) (P < 0.001) and peripheral blood (PB) (P < 0.0001), FLT3-internal tandem duplications (FLT3-ITD) (P = 0.010) and NPM1 mutations (P = 0.015). More importantly, reduced CXCL12 expression predicted worse overall survival (OS) and event-free survival (EFS) in all AML, non-M3-AML, and cytogenetically normal (CN)-AML patients in three independent cohorts. As for immune cell infiltration, high CXCL12 expressed groups tended to harbor more memory B cells and plasma cells infiltration while low CXCL12 expressed groups exhibited more eosinophils infiltration. GO enrichment and KEGG pathways analysis revealed the potential biological progress the gene participating in.

Conclusions

CXCL12 is significantly down-regulated in AML and low CXCL12 expression is an independent and poor predictor of AML prognosis. CXCL12 expression level correlates with clinical and immune characteristics of AML, which could provide potential assistance for treatment. Prospective studies are needed to further validate the impact of CXCL12 expression before routine clinical application in AML.

Leukemia stem cell-bone marrow microenvironment interplay in acute myeloid leukemia development

Despite the advances in intensive chemotherapy regimens and targeted therapies, overall survival (OS) of acute myeloid leukemia (AML) remains unfavorable due to inevitable chemotherapy resistance and high relapse rate, which mainly caused by the persistence existence of leukemia stem cells (LSCs). Bone marrow microenvironment (BMM), the home of hematopoiesis, has been considered to play a crucial role in both hematopoiesis and leukemogenesis. When interrupted by the AML cells, a malignant BMM formed and thus provided a refuge for LSCs and protecting them from the cytotoxic effects of chemotherapy. In this review, we summarized the alterations in the bidirectional interplay between hematopoietic cells and BMM in the normal/AML hematopoietic environment, and pointed out the key role of these alterations in pathogenesis and chemotherapy resistance of AML. Finally, we focused on the current potential BMM-targeted strategies together with future prospects and challenges. Accordingly, while further research is necessary to elucidate the underlying mechanisms behind LSC–BMM interaction, targeting the interaction is perceived as a potential therapeutic strategy to eradicate LSCs and ultimately improve the outcome of AML.

The Role of Hypoxic Bone Marrow Microenvironment in Acute Myeloid Leukemia and Future Therapeutic Opportunities

Acute myeloid leukemia (AML) is a hematologic malignancy caused by a wide range of alterations responsible for a high grade of heterogeneity among patients. Several studies have demonstrated that the hypoxic bone marrow microenvironment (BMM) plays a crucial role in AML pathogenesis and therapy response. This review article summarizes the current literature regarding the effects of the dynamic crosstalk between leukemic stem cells (LSCs) and hypoxic BMM. The interaction between LSCs and hypoxic BMM regulates fundamental cell fate decisions, including survival, self-renewal, and proliferation capacity as a consequence of genetic, transcriptional, and metabolic adaptation of LSCs mediated by hypoxia-inducible factors (HIFs). HIF-1α and some of their targets have been associated with poor prognosis in AML. It has been demonstrated that the hypoxic BMM creates a protective niche that mediates resistance to therapy. Therefore, we also highlight how hypoxia hallmarks might be targeted in the future to hit the leukemic population to improve AML patient outcomes.

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.

NK cell surveillance of hematological malignancies. Therapeutic implications and regulation by chemokine receptors

NK cells are circulating innate lymphoid cells that constantly move from bloodstream into tissues, exerting several functions including tumor surveillance. For this reason, NK cells are considered attractive target for cancer immunotherapy. Several strategies are employed to harness NK cell efficacy especially in hematological tumors, including adoptive transfer, genetic manipulation to overexpress chimeric antigen receptors and cytokine or immunomodulatory drug treatments of ex-vivo cultivated and expanded NK cells. Several chemokine receptors support NK cell tissue homing and are required for efficient tumor infiltration. Nevertheless, chemokine receptor expression is often insufficient, or their respective ligands may not be expressed in the tumor microenvironment, thus limiting NK cell localization at the tumor site. Therefore, strategies to implement expression or promote the function of the correct chemokine receptor/ligand axes have been employed in the last years with promising results in preclinical models. In this review, we discuss how chemokine receptors and their ligands regulate the trafficking and localization of NK cells in hematological tumors and how the chemokine function can be manipulated to improve current therapeutic approaches.

Antineoplastic effect of a diphtheria toxin-based nanoparticle targeting acute myeloid leukemia cells overexpressing CXCR4

Nanomedicine has opened an opportunity to improve current clinical practice by enhancing the selectivity in the delivery of antitumor drugs to specific cancer cells. These new strategies are able to bypass toxicity on normal cells increasing the effectiveness of current anticancer treatments. In acute myeloid leukemia (AML) current chemotherapy treatments generate a relevant toxic impact in normal cells and severe side effects or even patient death. In this study, we have designed a self-assembling protein nanoparticle, T22-DITOX-H6, which incorporates a ligand (T22) targeting CXCR4-overexpressing (CXCR4⁺) cells, and a potent cytotoxic diphtheria toxin domain. CXCR4 is overexpressed in AML leukemic cells and associates with poor prognosis, being, therefore, a relevant clinical target. We demonstrate here that T22-DITOX-H6 induces apoptosis in CXCR4⁺ leukemic cells through CXCR4-dependent internalization. In addition, repeated T22-DITOX-H6 treatment (10 μg/dose per 10 doses, intravenously injected) in a disseminated AML mouse model (NSG mice intravenously injected with THP-1-Luci cells, n = 10 per group) potently blocks the dissemination of AML cells in bone marrow, spleen and liver of treated mice, without inducing toxicity in healthy tissues. In conclusion, our strategy of selectively ablating CXCR4 positive leukemic cells by administering the T22-DITOX-H6 nanoparticle could be a promising treatment, especially in patients undergoing AML relapse after chemotherapy, in which leukemic cells overexpress CXCR4.

Hematopoietic versus leukemic stem cell quiescence: Challenges and therapeutic opportunities

Hematopoietic stem cells (HSC) are responsible for the production of mature blood cells. To ensure that the HSC pool does not get exhausted over the lifetime of an individual, most HSCs are in a state of quiescence with only a small proportion of HSCs dividing at any one time. HSC quiescence is carefully controlled by both intrinsic and extrinsic, niche-driven mechanisms. In acute myeloid leukemia (AML), the leukemic cells overtake the hematopoietic bone marrow niche where they acquire a quiescent state. These dormant AML cells are resistant to chemotherapeutics. Because they can re-establish the disease after therapy, they are often termed as quiescent leukemic stem cells (LSC) or leukemia-initiating cells. While advancements are being made to target particular driver mutations in AML, there is less focus on how to tackle the drug resistance of quiescent LSCs. This review summarises the current knowledge on the biochemical characteristics of quiescent HSCs and LSCs, the intracellular signaling pathways and the niche-driven mechanisms that control quiescence and the key differences between HSC- and LSC-quiescence that may be exploited for therapy.

Scaffold-mediated CRISPR-Cas9 delivery system for acute myeloid leukemia therapy

Leukemia stem cells (LSCs) sustain the disease and contribute to relapse in acute myeloid leukemia (AML). Therapies that ablate LSCs may increase the chance of eliminating this cancer in patients. To this end, we used a bioreducible lipidoid-encapsulated Cas9/single guide RNA (sgRNA) ribonucleoprotein [lipidoid nanoparticle (LNP)-Cas9 RNP] to target the critical gene interleukin-1 receptor accessory protein (IL1RAP) in human LSCs. To enhance LSC targeting, we loaded LNP-Cas9 RNP and the chemokine CXCL12α onto mesenchymal stem cell membrane-coated nanofibril (MSCM-NF) scaffolds mimicking the bone marrow microenvironment. In vitro, CXCL12α release induced migration of LSCs to the scaffolds, and LNP-Cas9 RNP induced efficient gene editing. IL1RAP knockout reduced LSC colony-forming capacity and leukemic burden. Scaffold-based delivery increased the retention time of LNP-Cas9 in the bone marrow cavity. Overall, sustained local delivery of Cas9/IL1RAP sgRNA via CXCL12α-loaded LNP/MSCM-NF scaffolds provides an effective strategy for attenuating LSC growth to improve AML therapy.

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.

Risk Stratification in Acute Myeloid Leukemia Using CXCR Gene Signatures: A Bioinformatics Analysis

The role of CXC chemokine receptors in tumors has been an increasingly researched focus in recent years. However, significant prognostic values of CXCR members in acute myeloid leukemia are yet to be explored profoundly. In this study, we firstly made an analysis of the relationship of CXCR family members and AML using samples from TCGA. Our results suggested that transcriptional expressions of CXCRs serve an important role in AML. CXCR transcript expressions, except CXCR1 expression, were significantly increased in AML. It displayed the expression pattern of CXCR members in different AML subtypes according to FAB classification. The correlations of CXCR transcript expression with different genotypes and karyotypes were also present. High CXCR2 expression was found to have a significantly worse prognosis compared with that of low CXCR2 expression, and CXCR2 was also found to be an independent prognostic factor. We also established a CXCR signature to identify high-risk subgroups of patients with AML. It was an independent prognostic factor and could become a powerful method to predict the survival rate of patients.

Studying leukemia stem cell properties and vulnerabilities with human iPSCs

The reprogramming of cancer cells into induced pluripotent stem cells (iPSCs) can capture entire cancer genomes, and thus create genetically faithful models of human cancers. By providing stringent genetically clonal conditions, iPSC modeling can also unveil non-genetic sources of cancer heterogeneity and provide a unique opportunity to study them separately from genetic sources, as we recently showed in an iPSC-based model of acute myeloid leukemia (AML). Genetically clonal iPSCs, derived from a patient with AML, reproduce, upon hematopoietic differentiation, phenotypic and functional heterogeneity with all the hallmarks of a leukemia stem cell (LSC) hierarchy. Here we discuss the lessons that can be learned about the LSC state, its plasticity, stability and genetic and epigenetic determinants from iPSC modeling. We also discuss the practical and translational implications of exploiting AML-iPSCs to prospectively isolate large numbers of iLSCs for large-scale experiments, such as screens, and for discovery of new therapeutic targets specific to AML LSCs.

BL-8040 CXCR4 antagonist is safe and demonstrates antileukemic activity in combination with cytarabine for the treatment of relapsed/refractory acute myelogenous leukemia: An open-label safety and efficacy phase 2a study

BACKGROUND

CXCR4 mediates the retention and survival of acute myelogenous leukemia blasts in bone marrow and contributes to their resistance to chemotherapy. The authors evaluated a combination of the high-affinity CXCR4 antagonist BL-8040 with high-dose cytarabine (HiDAC) chemotherapy in a phase 2a study of patients with relapsed and refractory AML.

METHODS

Forty-two patients received treatment with BL-8040 monotherapy for 2 days followed by a combination of BL-8040 with HiDAC for 5 days. Six escalating BL-8040 dose levels were investigated (0.5, 0.75, 1.0, 1.25, 1.5, and 2.0 mg/kg), and 1.5 mg/kg was selected as the dose for the expansion phase (n = 23).

RESULTS

BL-8040 in combination with HiDAC was safe and well tolerated at all dose levels. Clinical response was observed with BL-8040 doses ≥1.0 mg/kg. The composite response rate (complete remissions plus complete remissions with incomplete hematologic recovery of platelets or neutrophils) was 29% (12 of 42) in all patients and 39% (9 of 23) in the 1.5-mg/kg phase. The median overall survival was 8.4 months for all patients, 10.8 months in the 1.5-mg/kg phase, and 21.8 months for responding patients in the 1.5-mg/kg cohort. Two days of BL-8040 monotherapy triggered the mobilization of blasts into peripheral blood, with significantly higher mean fold-changes in responders versus nonresponders. This was accompanied by a decrease in bone marrow blasts.

CONCLUSIONS

The current results demonstrate the efficacy of CXCR4 targeting with BL-8040 and support continued clinical development in acute myelogenous leukemia.

Phase 1 study of combinatorial sorafenib, G-CSF, and plerixafor treatment in relapsed/refractory, FLT3-ITD-mutated acute myelogenous leukemia patients

Stroma-leukemia interactions mediated by CXCR4, CD44, VLA4, and their respective ligands contribute to therapy resistance in FLT3-ITD-mutated acute myelogenous leukemia (AML). We conducted a phase 1 study with the combination of sorafenib (a FLT3-ITD inhibitor), plerixafor (a SDF-1/CXCR4 inhibitor), and G-CSF (that cleaves SDF-1, CD44, and VLA4). Twenty-eight patients with relapsed/refractory FLT3-ITD-mutated AML were enrolled from December 2010 to December 2013 at three dose levels of sorafenib (400, 600, and 800 mg twice daily) and G-CSF and plerixafor were administered every other day for seven doses starting on day one. Sorafenib 800 mg twice daily was selected for the expansion phase. While no dose-limiting toxicities (DLT) were encountered in the four-week DLT window, hand-foot syndrome and rash were seen beyond the DLT window, which required dose reductions in most patients. The response rate was 36% (complete response (CR) = 4, complete remission with incomplete platelet recovery (CRp) = 4, complete remission with incomplete hematologic recovery (CRi) = 1, and partial response (PR) = 1) for the intention to treat population. Treatment resulted in 58.4 and 47 mean fold mobilization of blasts and CD34 /38- stem/progenitor cells, respectively, to the circulation. Expression of the adhesion molecules CXCR4, CD44, and VLA4 on circulating leukemia cells correlated negatively with the mobilization of CD34+/38-, CD34+/38-/123+ "progenitor" cells (all P ≤ .002). Mass cytometry analysis of sequential samples from two patients demonstrated resistance emerging early on from sub-clones with persistent Akt and/or ERK signaling. In conclusion, the strategy of combined inhibition of FLT3 kinase and stromal adhesive interactions has promising activity in relapsed/refractory, FLT3-ITD-mutated AML, which warrants further evaluation in the front-line setting.

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.

Targeting CXCR4 in AML and ALL

The interaction of acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) blasts with the bone marrow microenvironment regulates self-renewal, growth signaling, as well as chemotherapy resistance. The chemokine receptor, CXC receptor 4 (CXCR4), with its ligand chemokine ligand 12 (CXCL12), plays a key role in the survival and migration of normal and malignant stem cells to the bone marrow. High expression of CXCR4 on AML and ALL blasts has been shown to be a predictor of poor prognosis for these diseases. Several small molecule inhibitors, short peptides, antibodies, and antibody drug conjugates have been developed for the purposes of more effective targeting and killing of malignant cells expressing CXCR4. In this review we will discuss recent results and strategies in targeting CXCR4 with these agents in patients with AML or ALL.

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.

CXCR4 Inhibition Enhances Efficacy of FLT3 Inhibitors in FLT3-Mutated AML Augmented by Suppressed TGF-b Signaling

Given the proven importance of the CXCL12/CXCR4 axis in the stroma–acute myeloid leukemia (AML) interactions and the rapid emergence of resistance to FLT3 inhibitors, we investigated the efficacy and safety of a novel CXCR4 inhibitor, LY2510924, in combination with FLT3 inhibitors in preclinical models of AML with FLT3-ITD mutations (FLT3-ITD-AML). Quizartinib, a potent FLT3 inhibitor, induced apoptosis in FLT3-ITD-AML, while LY2510924 blocked surface CXCR4 without inducing apoptosis. LY2510924 significantly reversed stroma-mediated resistance against quizartinib mainly through the MAPK pathway. In mice with established FLT3-ITD-AML, LY2510924 induced durable mobilization and differentiation of leukemia cells, resulting in enhanced anti-leukemia effects when combined with quizartinib, whereas transient effects were seen on non-leukemic blood cells in immune-competent mice. Sequencing of the transcriptome of the leukemic cells surviving in vivo treatment with quizartinib and LY2510924 revealed that genes related to TGF-β signaling may confer resistance against the drug combination. In co-culture experiments of FLT3-ITD-AML and stromal cells, both silencing of TGF-β in stromal cells or TGF-β-receptor kinase inhibitor enhanced apoptosis by combined treatment. Disruption of the CXCL12/CXCR4 axis in FLT3-ITD-AML by LY2510924 and its negligible effects on normal immunocytes could safely enhance the potency of quizartinib, which may be further improved by blockade of TGF-β signaling.

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.

An Overview on G Protein-coupled Receptor-induced Signal Transduction in Acute Myeloid Leukemia

BACKGROUND

Acute Myeloid Leukemia (AML) is a genetically heterogeneous disease characterized by uncontrolled proliferation of precursor myeloid-lineage cells in the bone marrow. AML is also characterized by patients with poor long-term survival outcomes due to relapse. Many efforts have been made to understand the biological heterogeneity of AML and the challenges to develop new therapies are therefore enormous. G Protein-coupled Receptors (GPCRs) are a large attractive drug-targeted family of transmembrane proteins, and aberrant GPCR expression and GPCR-mediated signaling have been implicated in leukemogenesis of AML. This review aims to identify the molecular players of GPCR signaling, focusing on the hematopoietic system, which are involved in AML to help developing novel drug targets and therapeutic strategies.

METHODS

We undertook an exhaustive and structured search of bibliographic databases for research focusing on GPCR, GPCR signaling and expression in AML.

RESULTS AND CONCLUSION

Many scientific reports were found with compelling evidence for the involvement of aberrant GPCR expression and perturbed GPCR-mediated signaling in the development of AML. The comprehensive analysis of GPCR in AML provides potential clinical biomarkers for prognostication, disease monitoring and therapeutic guidance. It will also help to provide marker panels for monitoring in AML. We conclude that GPCR-mediated signaling is contributing to leukemogenesis of AML, and postulate that mass spectrometrybased protein profiling of primary AML cells will accelerate the discovery of potential GPCR related biomarkers for AML.

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.

Cancer Stem Cells in the Development of Novel Therapeutics for Refractory Pediatric Leukemia

Although treatment strategies for pediatric leukemia have improved overall survival rates in the recent past, relapse rates in certain subgroups such as infant leukemia remain unacceptably high. Despite undergoing extensive chemotherapy designed to target the rapidly proliferating leukemia cells, many of these children experience relapse. In refractory leukemia, the existence of cell populations with stemness characteristics, termed leukemia stem cells (LSCs), which remain quiescent and subsequently replenish the blast population, has been described. A significant body of evidence exists, derived largely from xenograft models of adult acute myeloid leukemia, to support the idea that LSCs may play a fundamental role in refractory disease. In addition, clinical studies have also linked LSCs with increased minimal residual disease, higher relapse rate, and decreased survival rates in these patients. Recently, a number of reports have addressed effective ways to utilize new-generation genomic sequencing and transcriptomic analyses to identify targeted therapeutic agents aimed at LSCs, while sparing normal hematopoietic stem cells. These data underscore the value of timely translation of knowledge from adult studies to the unique molecular and physiological characteristics seen in pediatric leukemia. We aim to summarize this article in the rapidly expanding field of stem cell biology in hematopoietic malignancies, focusing particularly on relevant preclinical models and novel targeted therapeutics, and their applicability to childhood leukemia.

Prognostic significance of CXCR4 expression in acute myeloid leukemia

Background

CXCR4 chemokine receptors play an important role in leukemia proliferation, extramedullary migration, infiltration, adhesion, and resistance to chemotherapy drugs.

Methods

The CXCR4 expression by flow cytometry in 122 acute myeloid leukemia (AML) patients between 2010 and 2014 was analyzed.

Results

The expression of CXCR4 in AML‐M4/M5 was found to be significantly higher than that of other subtypes according to both FAB subtype and WHO classification. The FLT3‐ITD mutant was significantly higher in high CXCR4 expression group (P = .0086). Our data also showed that CXCR4 expression was correlated with CD64 expression. Low CXCR4 expression on AML cells was associated with better prognosis, and the median overall survival (OS) for low CXCR4 expression patients was 318 days, compared with 206 days for patients with high CXCR4 expression (P = .045). Multivariate analysis revealed that CXCR4 expression, age, and extramedullary infiltration were independent prognostic factors.

Conclusions

Our study demonstrated that CXCR4 expression in AML was an independent prognostic predictor for disease survival that could be rapidly and easily determined by flow cytometry at disease presentation.

Bone marrow microenvironment: The guardian of leukemia stem cells

Bone marrow microenvironment (BMM) is the main sanctuary of leukemic stem cells (LSCs) and protects these cells against conventional therapies. However, it may open up an opportunity to target LSCs by breaking the close connection between LSCs and the BMM. The elimination of LSCs is of high importance, since they follow cancer stem cell theory as a part of this population. Based on cancer stem cell theory, a cell with stem cell-like features stands at the apex of the hierarchy and produces a heterogeneous population and governs the disease. Secretion of cytokines, chemokines, and extracellular vesicles, whether through autocrine or paracrine mechanisms by activation of downstream signaling pathways in LSCs, favors their persistence and makes the BMM less hospitable for normal stem cells. While all details about the interactions of the BMM and LSCs remain to be elucidated, some clinical trials have been designed to limit these reciprocal interactions to cure leukemia more effectively. In this review, we focus on chronic myeloid leukemia and acute myeloid leukemia LSCs and their milieu in the bone marrow, how to segregate them from the normal compartment, and finally the possible ways to eliminate these cells.

The FLT3-ITD mutation and the expression of its downstream signaling intermediates STAT5 and Pim-1 are positively correlated with CXCR4 expression in patients with acute myeloid leukemia

Chemokine ligand 12(CXCL12) mediates signaling through chemokine receptor 4(CXCR4), which is essential for the homing and maintenance of Hematopoietic stem cells (HSCs) in the bone marrow. FLT3-ITD mutations enhance cell migration toward CXCL12, providing a drug resistance mechanism underlying the poor effects of FLT3-ITD antagonists. However, the mechanism by which FLT3-ITD mutations regulate the CXCL12/CXCR4 axis remains unclear. We analyzed the relationship between CXCR4 expression and the FLT3-ITD mutation in 466 patients with de novo AML to clarify the effect of FLT3-ITD mutations on CXCR4 expression in patients with AML. Our results indicated a positive correlation between the FLT3-ITD mutant-type allelic ratio (FLT3-ITD MR) and the relative fluorescence intensity (RFI) of CXCR4 expression in patients with AML (r = 0.588, P ≤ 0.0001). Moreover, the levels of phospho(p)-STAT5, Pim-1 and CXCR4 proteins were positively correlated with the FLT3-ITD MR, and the mRNA levels of CXCR4 and Pim-1 which has been revealed as one of the first known target genes of STAT5, were upregulated with an increasing FLT3-ITD MR(P < 0.05). Therefore, FLT3-ITD mutations upregulate the expression of CXCR4 in patients with AML, and the downstream signaling intermediates STAT5 and Pim-1 are also involved in this phenomenon and subsequently contribute to chemotherapy resistance and disease relapse in patients with AML. However, the mechanism must be confirmed in further experiments. The combination of CXCR4 antagonists and FLT3 inhibitors may improve the sensitivity of AML cells to chemotherapy and overcome drug resistance.

Targeting Leukemia Stem Cell-Niche Dynamics: A New Challenge in AML Treatment

One of the most urgent needs in AML is to improve the disease cure rate as relapse still occurs in 60–80% of patients. Recent evidence suggests that dismal clinical outcomes may be improved by a better definition of the tight interaction between the AML cell population and the bone marrow (BM) microenvironment (“the niche”); the latter has been progressively highlighted to have an active role in the disease process. It has now been well established that the leukemic population may misinterpret niche-derived signals and remodel the niche, providing a shelter to AML cells and protecting them from the cytotoxic effects of chemoradiotherapy. Novel imaging technological advances and preclinical disease models have revealed that, due to the finite number of BM niches, leukemic stem cells (LSCs) and normal hematopoietic stem cells (HSCs) compete for the same functional areas. Thus, the removal of LSCs from the BM niche and the promotion of normal HSC engraftment should be the primary goals in antileukemic research. In addition, it is now becoming increasingly clear that AML-niche dynamics are disease stage specific. In AML, the niche has been linked to disease pathogenesis in the preleukemic stage, the niche becomes permissive once leukemic cells are established, and the niche is transformed into a self-reinforcing structure at a later disease stage. These concepts have been fostered by the demonstration that, in unrelated AML types, endosteal vessel loss occurs as a primary AML-induced niche alteration, and additional AML-induced alterations of the niche and normal hematopoiesis evolve focally and in parallel. Obviously, this endosteal vessel loss plays a fundamental role in AML pathogenesis by causing excessive vascular permeability, hypoxia, altered perfusion, and reduced drug delivery. Each of these alterations may be effectively targeted by various therapeutic procedures, but preservation of endosteal vessel integrity might be the best option for any future antileukemic treatment.

Relationship between CXC chemokine receptor 4 expression and prognostic significance in acute myeloid leukemia

CXC chemokine receptor 4 (CXCR4) expression on acute myeloid leukemia (AML) cells correlated with stromal cell derived factor-1α (SDF-1α) and retained hematopoietic progenitors and leukemia cells within the bone marrow microenvironment. Here, we examined CXCR4 expression in 134 de novo AML and 21 controls by flow cytometry, evaluated the relationship between CXCR4 expression and clinical characteristics, and elucidated the prognostic significance of CXCR4 expression in AML prospectively. We found that the CXCR4 expression was significantly higher in AML patients than controls (P = .000). One hundred thirty four cases of de novo AML patients were divided into 2 groups according to the median of CXCR4 relative fluorescence intensity (RFI). CXCR4 high group (RFI >4.23) had markedly shorter overall survival (OS) and disease-free survival (DFS) than CXCR4 low group (RFI ≤4.23) in 106 AML patients who received chemotherapy (P = .002; .026, respectively). Furthermore, in the 87 non-M3 patients who received induction therapy, there was a significant decrease for OS but not for DFS in the CXCR4 high group (P = .047 and .178, respectively). Moreover, high levels of CXCR4 expression independently increased the risk of relapse in both all AML and non-M3 patients who achieved complete remission (CR) after chemotherapy (odds ratio = 1.090, P = .010; odds ratio = 1.068, P = .048, respectively). Collectively, our data suggest that CXCR4 overexpression was an independent prognostic factor for disease relapse and poorer OS in both all AML and non-M3 patients. CXCR4 expression levels can be determined at disease presentation by the flow rapidly and easily. As such, CXCR4 could be used as a potential therapeutic target in AML patients with poor prognosis.

The role of cell adhesion in hematopoiesis and leukemogenesis

The cells of the bone marrow microenvironment are emerging as important contributors and regulators of normal hematopoiesis. This microenvironment is perturbed during leukemogenesis, and evidence points toward a bidirectional communication between leukemia cells and the normal cells of the bone marrow, mediated by direct cell-cell contact as well as soluble factors. These interactions are increasingly appreciated to play a role in leukemogenesis and possibly in resistance to chemotherapy. In fact, several compounds that specifically target the bone marrow microenvironment, including inhibitors of cell adhesion, are being tested as adjuncts to leukemia therapy.

Adhesion Deregulation in Acute Myeloid Leukaemia

Cell adhesion is a process through which cells interact with and attach to neighboring cells or matrix using specialized surface cell adhesion molecules (AMs). Adhesion plays an important role in normal haematopoiesis and in acute myeloid leukaemia (AML). AML blasts express many of the AMs identified on normal haematopoietic precursors. Differential expression of AMs between normal haematopoietic cells and leukaemic blasts has been documented to a variable extent, likely reflecting the heterogeneity of the disease. AMs govern a variety of processes within the bone marrow (BM), such as migration, homing, and quiescence. AML blasts home to the BM, as the AM-mediated interaction with the niche protects them from chemotherapeutic agents. On the contrary, they detach from the niches and move from the BM into the peripheral blood to colonize other sites, i.e., the spleen and liver, possibly in a process that is reminiscent of epithelial-to-mesenchymal-transition in metastatic solid cancers. The expression of AMs has a prognostic impact and there are ongoing efforts to therapeutically target adhesion in the fight against leukaemia.

Moving Myeloid Leukemia Drug Discovery Into the Third Dimension

The development of therapies aimed at leukemia has progressed substantially in the past years but childhood acute myeloid leukemia (AML) remains one of the most challenging cancers to treat. Genomic profiling of AML has greatly enhanced our understanding of the genetic and epigenetic landscape of this high-risk leukemia. With it comes the opportunity to develop targeted therapies that are expected to be more effective and less toxic than current treatment regimens. Nevertheless, often overlooked in leukemia drug discovery are the dynamic interactions between leukemic cells and the bone marrow environment. The interplay between leukemic cells, stromal cells and the extracellular matrix plays critical roles in the development, progression and relapse of AML as well as in drug response and the development of resistance. Here we will review pediatric leukemia with a special focus on acute myeloid disease in children, and discuss the tumor microenvironment in the context of drug resistance and leukemia stem cell survival. We will emphasize how three-dimensional (3D) cell-based drug discovery may offer hope for both the identification and advancement of more effective treatment options for patients suffering from this devastating disease.