Polo‐like‐kinase 1 (PLK1) as a molecular target to overcome SYK‐mediated resistance of B‐lineage acute lymphoblastic leukaemia cells to oxidative stress

SYK tyrosine kinase has emerged as a master regulator of cellular resistance to oxidative stress (OS) by mediating the activation of the anti‐apoptotic nuclear factor κB and phosphatidylinositol‐3 kinase/AKT pathways after OS exposure. Here, we present unprecedented experimental evidence that polo‐like kinase 1 (PLK1) is the upstream regulator of SYK in B‐lineage acute lymphoblastic leukaemia (ALL) cells. Selective inhibition of PLK‐1 with the leflunomide metabolite analogue α‐cyano‐β‐hydroxy‐β‐methyl‐N‐[4‐(trifluoromethoxy) phenyl]‐propenamide/LFM‐A12 abolished the resistance of B‐lineage ALL cells to OS by preventing the activation of the anti‐apoptotic SYK signal transduction pathway. Notably, LFM‐A12 treatments at non‐cytotoxic concentrations resulted in marked augmentation of clonogenic death in resistant human B‐lineage ALL cell lines challenged with OS. Further, LFM‐A12 augmented OS‐induced apoptosis of chemotherapy‐resistant primary leukaemic cells from relapsed B‐lineage ALL patients in vitro and markedly potentiated the in vivo anti‐leukaemic activity of total body irradiation (TBI) against leukaemia‐initiating cells in severe combined immunodeficient mouse xenograft models of B‐lineage ALL. This study is the first to identify PLK1 as a regulator of SYK tyrosine kinase and a molecular target to overcome SYK‐mediated resistance of B‐lineage ALL cells to OS.     

Lymphotoxin‐β receptor in microenvironmental cells promotes the development of T‐cell acute lymphoblastic leukaemia with cortical/mature immunophenotype

Lymphotoxin‐mediated activation of the lymphotoxin‐β receptor (LTβR; LTBR) has been implicated in cancer, but its role in T‐cell acute lymphoblastic leukaemia (T‐ALL) has remained elusive. Here we show that the genes encoding lymphotoxin (LT)‐α and LTβ (LTA, LTB) are expressed in T‐ALL patient samples, mostly of the TAL/LMO molecular subtype, and in the TEL‐JAK2 transgenic mouse model of cortical/mature T‐ALL (Lta, Ltb). In these mice, expression of Lta and Ltb is elevated in early stage T‐ALL. Surface LTα₁β₂ protein is expressed in primary mouse T‐ALL cells, but only in the absence of microenvironmental LTβR interaction. Indeed, surface LT expression is suppressed in leukaemic cells contacting Ltbr‐expressing but not Ltbr‐deficient stromal cells, both in vitro and in vivo, thus indicating that dynamic surface LT expression in leukaemic cells depends on interaction with its receptor. Supporting the notion that LT signalling plays a role in T‐ALL, inactivation of Ltbr results in a significant delay in TEL‐JAK2‐induced leukaemia onset. Moreover, young asymptomatic TEL‐JAK2;Ltbr^?/? mice present markedly less leukaemic thymocytes than age‐matched TEL‐JAK2;Ltbr^+/+ mice and interference with LTβR function at this early stage delayed T‐ALL development. We conclude that LT expression by T‐ALL cells activates LTβR signalling in thymic stromal cells, thus promoting leukaemogenesis.     

Antibody therapy for acute myeloid leukaemia

Novel therapies with increased efficacy and decreased toxicity are desperately needed for the treatment of acute myeloid leukaemia (AML). The anti CD33 immunoconjugate, gemtuzumab ozogamicin (GO), was withdrawn with concerns over induction mortality and lack of efficacy. However a number of recent trials suggest that, particularly in AML with favourable cytogenetics, GO may improve overall survival. This data and the development of alternative novel monoclonal antibodies (mAb) have renewed interest in the area. Leukaemic stem cells (LSC) are identified as the subset of AML blasts that reproduces the leukaemic phenotype upon transplantation into immunosuppressed mice. AML relapse may be caused by chemoresistant LSC and this has refocused interest on identifying and targeting antigens specific for LSC. Several mAb have been developed that target LSC effectively in xenogeneic models but only a few have begun clinical evaluation. Antibody engineering may improve the activity of potential new therapeutics for AML. The encouraging results seen with bispecific T cell‐engaging mAb‐based molecules against CD19 in the treatment of B‐cell acute lymphobalstic leukaemia, highlight the potential efficacy of engineered antibodies in the treatment of acute leukaemia. Potent engineered mAb, possibly targeting novel LSC antigens, offer hope for improving the current poor prognosis for AML.     

Targeting of acute myeloid leukaemia by cytokine‐induced killer cells redirected with a novel CD123‐specific chimeric antigen receptor

Current therapeutic regimens for acute myeloid leukaemia (AML) are still associated with high rates of relapse. Immunotherapy with T‐cells genetically modified to express chimeric antigen receptors (CARs) represents an innovative approach. Here we investigated the targeting of the interleukin three receptor alpha (IL3RA; CD123) molecule, which is overexpressed on AML bulk population, CD34⁺ leukaemia progenitors, and leukaemia stem cells (LSC) compared to normal haematopoietic stem/progenitor cells (HSPCs), and whose overexpression is associated with poor prognosis. Cytokine‐induced killer (CIK) cells were transduced with SFG‐retroviral‐vector encoding an anti‐CD123 CAR. Transduced cells were able to strongly kill CD123⁺ cell lines, as well as primary AML blasts. Interestingly, secondary colony experiments demonstrated that anti‐CD123.CAR preserved in vitro HSPCs, in contrast to a previously generated anti‐CD33.CAR, while keeping an identical cytotoxicity profile towards AML. Furthermore, limited killing of normal monocytes and CD123‐low‐expressing endothelial cells was noted, thus indicating a low toxicity profile of the anti‐CD123.CAR. Taken together, our results indicate that CD123‐specific CARs strongly enhance anti‐AML CIK functions, while sparing HSPCs and normal low‐expressing antigen cells, paving the way to develop novel immunotherapy approaches for AML treatment.     

Acute myeloid/T‐lymphoblastic leukaemia (AMTL): a distinct category of acute leukaemias with common pathogenesis in need of improved therapy

Advances in the classification of acute leukaemias have led to improved outcomes for a substantial fraction of patients. However, chemotherapy resistance remains a major problem for specific subsets of acute leukaemias. Here, we propose that a molecularly distinct subtype of acute leukaemia with shared myeloid and T cell lymphoblastic features, which we term acute myeloid/T‐lymphoblastic leukaemia (AMTL), is divided across 3 diagnostic categories owing to variable expression of markers deemed to be defining of myeloid and T‐lymphoid lineages, such as myeloperoxidase and CD3. This proposed diagnostic group is supported by (i) retained myeloid differentiation potential during early T cell lymphoid development, (ii) recognition that some cases of acute myeloid leukaemia (AML) harbour hallmarks of T cell development, such as T‐cell receptor gene rearrangements and (iii) common gene mutations in subsets of AML and T cell acute lymphoblastic leukaemia (T‐ALL), including WT1, PHF6, RUNX1 and BCL11B. This proposed diagnostic entity overlaps with early T cell precursor (ETP) T‐ALL and T cell/myeloid mixed phenotype acute leukaemias (MPALs), and also includes a subset of leukaemias currently classified as AML with features of T‐lymphoblastic development. The proposed classification of AMTL as a distinct entity would enable more precise prospective diagnosis and permit the development of improved therapies for patients whose treatment is inadequate with current approaches.     

Anti‐leukaemic activity of the TYK2 selective inhibitor NDI‐031301 in T‐cell acute lymphoblastic leukaemia

Activation of tyrosine kinase 2 (TYK2) contributes to the aberrant survival of T‐cell acute lymphoblastic leukaemia (T‐ALL) cells. Here we demonstrate the anti‐leukaemic activity of a novel TYK2 inhibitor, NDI‐031301. NDI‐031301 is a potent and selective inhibitor of TYK2 that induced robust growth inhibition of human T‐ALL cell lines. NDI‐031301 treatment of human T‐ALL cell lines resulted in induction of apoptosis that was not observed with the JAK inhibitors tofacitinib and baricitinib. Further investigation revealed that NDI‐031301 treatment uniquely leads to activation of three mitogen‐activated protein kinases (MAPKs), resulting in phosphorylation of ERK, SAPK/JNK and p38 MAPK coincident with PARP cleavage. Activation of p38 MAPK occurred within 1 h of NDI‐031301 treatment and was responsible for NDI‐031301‐induced T‐ALL cell death, as pharmacological inhibition of p38 MAPK partially rescued apoptosis induced by TYK2 inhibitor. Finally, daily oral administration of NDI‐031301 at 100 mg/kg bid to immunodeficient mice engrafted with KOPT‐K1 T‐ALL cells was well tolerated, and led to decreased tumour burden and a significant survival benefit. These results support selective inhibition of TYK2 as a promising potential therapeutic strategy for T‐ALL.     

Effects of pharmacological and genetic disruption of CXCR4 chemokine receptor function in B‐cell acute lymphoblastic leukaemia

B cell acute lymphoblastic leukaemia (B‐ALL) cells express high levels of CXCR4 chemokine receptors for homing and retention within the marrow microenvironment. Bone marrow stromal cells (BMSC) secrete CXCL12, the ligand for CXCR4, and protect B‐ALL cells from cytotoxic drugs. Therefore, the therapeutic use of CXCR4 antagonists has been proposed to disrupt cross talk between B‐ALL cells and the protective stroma. Because CXCR4 antagonists can have activating agonistic function, we compared the genetic and pharmacological deletion of CXCR4 in B‐ALL cells, using CRISPR‐Cas9 gene editing and CXCR4 antagonists that are in clinical use (plerixafor, BKT140). Both genetic and pharmacological CXCR4 inhibition significantly reduced B‐ALL cell migration to CXCL12 gradients and beneath BMSC, and restored drug sensitivity to dexamethasone, vincristine and cyclophosphamide. NOD/SCID/IL‐2rγnull mice injected with CXCR4 gene‐deleted B‐ALL cells had significant delay in disease progression and superior survival when compared to control mice injected with CXCR4 wild‐type B‐ALL cells. These findings indicate that anti‐leukaemia activity of CXCR4 antagonists is primarily due to CXCR4 inhibition, rather than agonistic activity, and corroborate that CXCR4 is an important target to overcome stroma‐mediated drug resistance in B‐ALL.     

HMGA2 as a potential molecular target in KMT2A‐AFF1‐positive infant acute lymphoblastic leukaemia

Acute lymphoblastic leukaemia (ALL) in infants is an intractable cancer in childhood. Although recent intensive chemotherapy progress has considerably improved ALL treatment outcome, disease cure is often accompanied by undesirable long‐term side effects, and efficient, less toxic molecular targeting therapies have been anticipated. In infant ALL cells with KMT2A (MLL) fusion, the microRNA let‐7b (MIRLET7B) is significantly downregulated by DNA hypermethylation of its promoter region. We show here that the expression of HMGA2, one of the oncogenes repressed by MIRLET7B, is reversely upregulated in infant ALL leukaemic cells, particularly in KMT2A‐AFF1 (MLL‐AF4) positive ALL. In addition to the suppression of MIRLET7B, KMT2A fusion proteins positively regulate the expression of HMGA2. HMGA2 is one of the negative regulators of CDKN2A gene, which encodes the cyclin‐dependent kinase inhibitor p16^INK4A. The HMGA2 inhibitor netropsin, when combined with demethylating agent 5‐azacytidine, upregulated and sustained the expression of CDKN2A, which resulted in growth suppression of KMT2A‐AFF1‐expressing cell lines. This effect was more apparent compared to treatment with 5‐azacytidine alone. These results indicate that the MIRLET7B‐HMGA2‐CDKN2A axis plays an important role in cell proliferation of leukaemic cells and could be a possible molecular target for the therapy of infant ALL with KMT2A‐AFF1.     

A recombinant trispecific single‐chain Fv derivative directed against CD123 and CD33 mediates effective elimination of acute myeloid leukaemia cells by dual targeting

Two trivalent constructs consisting of single‐chain Fv antibody fragments (scFvs) specific for the interleukin‐3 receptor α chain (CD123), CD33 and the Fcγ‐receptor III (CD16) were designed and characterized for the elimination of acute myeloid leukaemia (AML) cells. The dual targeting single‐chain Fv triplebody (sctb) [123 × ds16 × 33] and the mono targeting sctb [123 × ds16 × 123] both specifically bound their respective target antigens and were stable in human serum at 37°C for at least 5 d. Both constructs induced potent antibody‐dependent cellular cytotoxicity (ADCC) of two different AML‐derived CD33‐ and CD123 double‐positive cell lines in the low picomolar range using isolated mononuclear cells (MNCs) as effector cells. In these experiments the dual targeting molecule produced significantly stronger lysis than the mono targeting agent. In addition, the sctbs showed a high potency in mediating ADCC of primary leukaemia cells isolated from peripheral blood or bone marrow of seven AML patients. Hence, these novel molecules displayed potent anti‐leukaemic effects against AML cells in vitro and represent attractive candidates for further preclinical development.     

CD22ΔE12 as a molecular target for RNAi therapy

B‐precursor acute lymphoblastic leukaemia (BPL) is the most common form of cancer in children and adolescents. Our recent studies have demonstrated that CD22ΔE12 is a characteristic genetic defect of therapy‐refractory clones in paediatric BPL and implicated the CD22ΔE12 genetic defect in the aggressive biology of relapsed or therapy‐refractory paediatric BPL. The purpose of the present study is to evaluate the biological significance of the CD22ΔE12 molecular lesion in BPL and determine if it could serve as a molecular target for RNA interference (RNAi) therapy. Here we report a previously unrecognized causal link between CD22ΔE12 and aggressive biology of human BPL cells by demonstrating that siRNA‐mediated knockdown of CD22ΔE12 in primary leukaemic B‐cell precursors is associated with a marked inhibition of their clonogenicity. Additionally, we report a nanoscale liposomal formulation of CD22ΔE12‐specific siRNA with potent in vitro and in vivo anti‐leukaemic activity against primary human BPL cells as a first‐in‐class RNAi therapeutic candidate targeting CD22ΔE12.     

Prognostic factors for acute myeloid leukaemia in adults – biological significance and clinical use

Acute myeloid leukaemia (AML) is a heterogenous disease. Prognosis of AML is influenced both by patient‐specific as well as disease‐specific factors. Age is the most prominent patient‐specific risk factor, while chromosomal aberrations are the strongest disease‐specific risk factors. For patients with cytogenetically normal AML, prognosis can be specified by mutational status of the genes NPM1, FLT3 and CEBPA. A growing number of recurrent mutations in additional genes have recently been identified, for which the prognostic effect yet has to be determined. Performance status, geriatric assessment, secondary leukaemia following myelodysplastic syndrome or cytotoxic treatment, common laboratory parameters, leukaemic stem cell frequency, bone marrow microenvironment, gene expression levels, epigenetic changes, micro‐RNA's as well as kinetics and depth of response to treatment influence prognosis of AML patients. Despite the high number of established risk factors, only few predictive markers exist which can truly aid therapy decisions in patients with AML.     

Significant cytotoxic activity in vitro of the EGFR tyrosine kinase inhibitor gefitinib in acute myeloblastic leukaemia

Objectives: Gefitinib inhibits epidermal growth factor receptor (EGFR) signalling, but may also act by non‐EGFR dependent mechanisms. We have investigated the activity of gefitinib in haematological tumour cells, in particular acute myeloblastic leukaemia (AML). Methods: Cytotoxic activity of gefitinib, alone or in combination with standard anti‐leukaemic drugs, was assessed by the short‐term fluorometric microculture cytotoxicity assay in tumour cells from 117 patients representing five haematological and five non‐haematological malignancies. In AML, the EGFR status was analysed by immunochemistry. Gefitinib‐induced apoptosis was investigated in a subset of AML samples, as well as in the leukaemia cell line MV‐4‐11, using a multiparametric high content screening assay. To confirm activation of caspase‐3 in cells treated with gefitinib, a blocking test was carried out in which MV4‐11 cells were pretreated with the specific caspase inhibitor DEVD‐FMK. Results: Gefitinib showed highest cytotoxic activity in AML (n = 19) with many samples being sensitive at concentrations achievable in clinical practice (<10 μM ), and no difference between previously untreated and relapsed patients. No correlation between the activity of gefitinib and standard antileukaemic drugs (cytarabine, doxorubicin, etoposide) was observed. Combining gefitinib with these drugs resulted in mainly additive or synergistic (etoposide) effects, with no evidence of sequence dependency. The AML cells did not express the EGFR. Gefitinib induced apoptosis, which was at least partly mediated by activation of the caspase‐3 pathway. Conclusion: In vitro, gefitinib has significant cytotoxic activity in AML by inducing apoptosis through non‐EGFR dependent pathways.     

Disturbed CXCR4/CXCL12 axis in paediatric precursor B‐cell acute lymphoblastic leukaemia

Malignant cells infiltrating the bone marrow (BM) interfere with normal cellular behaviour of supporting cells, thereby creating a malignant niche. We found that CXCR4‐receptor expression was increased in paediatric precursor B‐cell acute lymphoblastic leukaemia (BCP‐ALL) cells compared with normal mononuclear haematopoietic cells (P < 0·0001). Furthermore, high CXCR4‐expression correlated with an unfavourable outcome in BCP‐ALL (5‐year cumulative incidence of relapse ± standard error: 38·4% ± 6·9% in CXCR4‐high versus 12% ± 4·6% in CXCR4‐low expressing cases, P < 0·0001). Interestingly, BM levels of the CXCR4‐ligand (CXCL12) were 2·7‐fold lower (P = 0·005) in diagnostic BCP‐ALL samples compared with non‐leukaemic controls. Induction chemotherapy restored CXCL12 levels to normal. Blocking the CXCR4‐receptor with Plerixafor showed that the lower CXCL12 serum levels at diagnosis could not be explained by consumption by the leukaemic cells, nor did we observe an altered CXCL12‐production capacity of BM‐mesenchymal stromal cells (BM‐MSC) at this time‐point. We rather observed that a very high density of leukaemic cells negatively affected CXCL12‐production by the BM‐MSC while stimulating the secretion levels of granulocyte colony‐stimulating factor (G‐CSF). These results suggest that highly proliferative leukaemic cells are able to down‐regulate secretion of cytokines involved in homing (CXCL12), while simultaneously up‐regulating those involved in haematopoietic mobilization (G‐CSF). Therefore, interference with the CXCR4/CXCL12 axis may be an effective way to mobilize BCP‐ALL cells.     

Antibody‐based therapies in B‐cell lineage acute lymphoblastic leukaemia

Targeted therapies represent a major breakthrough in the treatment of adult acute lymphoblastic leukaemia (ALL). Because lymphoblastic leukaemia cells express a variety of specific antigens, those ones can serve as targets for monoclonal antibodies (MoAbs). Anti‐CD20 (rituximab), anti‐CD19 (blinatumomab, SAR3419), anti‐CD22 (epratuzumab, inotuzumab ozogamicin) and anti‐CD52 (alemtuzumab) have therefore been developed. Possible strategies even include recruitment of CD3 cytotoxic T cells (blinatumomab) or adoptive T‐cell therapy by gene transfer of CD19‐chimeric antigen receptors (CD19‐CARs). Recent data show that antibody‐based therapy is a highly promising treatment approach. However, optimal treatment approach still needs to be defined.     

Upregulation of AKR1C1 in mesenchymal stromal cells promotes the survival of acute myeloid leukaemia cells

The leukaemic bone marrow microenvironment, comprising abnormal mesenchymal stromal cells (MSCs), is responsible for the poor prognosis of acute myeloid leukaemia (AML). Therefore, it is essential to determine the mechanisms underlying the supportive role of MSCs in the survival of leukaemia cells. Through in silico analyses, we identified a total of 271 aberrantly expressed genes in the MSCs derived from acute myeloid leukemia (AML) patients that were associated with adipogenic differentiation, of which aldo-keto reductase 1C1 (AKR1C1) was significantly upregulated in the AML-MSCs. Knockdown of AKR1C1 in the MSCs suppressed adipogenesis and promoted osteogenesis, and inhibited the growth of co-cultured AML cell lines compared to the situation in wild- type AML-derived MSCs. Introduction of recombinant human AKR1C1 in the MSCs partially alleviated the effects of AKR1C1 knockdown. In addition, the absence of AKR1C1 reduced secretion of cytokines such as MCP-1, IL-6 and G-CSF from the MSCs, along with inactivation of STAT3 and ERK1/2 in the co-cultured AML cells. AKR1C1 is an essential factor driving the adipogenic differentiation of leukaemic MSCs and mediates its pro-survival effects on AML cells by promoting cytokine secretion and activating the downstream pathways in the AML cells.     

Expression of an abnormal sized c-kit transcript in Hong Kong Chinese acute lymphoblastic leukaemia patients

Summary Blast cells from a majority of acute myelogenous leukaemia (AML) patients express c-kit mRNA. However, c-kit expression has not been observed in patients with acute lymphoblastic leukaemia (ALL) and lymphoproliferative disease. We report here the detection of an abnormal sized c-kit mRNA in two Hong Kong Chinese patients with pre-B ALL and common ALL.