FLT3 inhibition: a moving and evolving target in acute myeloid leukaemia. Leukemia. 2013;27:260-268. [PMID: 22797419 DOI: 10.1038/leu.2012.195]

FLT3 inhibitors: clinical potential in acute myeloid leukemia

Acute myeloid leukemia (AML) is an aggressive hematopoietic malignancy that is cured in as few as 15%–40% of cases. Tremendous improvements in AML prognostication arose from a comprehensive analysis of leukemia cell genomes. Among normal karyotype AML cases, mutations in the FLT3 gene are the ones most commonly detected as having a deleterious prognostic impact. FLT3 is a transmembrane tyrosine kinase receptor, and alterations of the FLT3 gene such as internal tandem duplications (FLT3-ITD) deregulate FLT3 downstream signaling pathways in favor of increased cell proliferation and survival. FLT3 tyrosine kinase inhibitors (TKI) emerged as a new therapeutic option in FLT3-ITD AML, and clinical trials are ongoing with a variety of TKI either alone, combined with chemotherapy, or even as maintenance after allogenic stem cell transplantation. However, a wide range of molecular resistance mechanisms are activated upon TKI therapy, thus limiting their clinical impact. Massive research efforts are now ongoing to develop more efficient FLT3 TKI and/or new therapies targeting these resistance mechanisms to improve the prognosis of FLT3-ITD AML patients in the future.

[The clinical and prognostic significance of acute myeloid leukemia with FLT3-ITD]

Objective: To investigate the clinical significance and prognosis factors of acute myeloid leukemia (AML) with FLT3 mutation-internal tandem duplication (FLT3-ITD) and provide more evidence for prognosis evaluation in AML with FLT3-ITD. Methods: The clinical characteristics and outcomes of 98 AML with FLT3-ITD were analyzed. Results: In patients with FLT3-ITD positive AML, the median of WBC and peripheral blood blast at initial diagnosis were 58.2 (0.3-461.8) ×109/L and 42.2 (0-397.1) ×109/L, respectively. The complete remission (CR) rate after one course induction therapy was 60.6% in 71 patients with intermediate or adverse risk. The primary refractory rate was 26.8% in 71 patients with intermediate or adverse risk. The lower number of peripheral blood blast was a favorable factor to achieve CR after one course induction therapy (P=0.009). CR after one course (HR=0.395, 95% CI 0.183-0.85, P=0.001) and allo-transplantation in CR1 (HR=0.180, 95% CI 0.043-0.752, P=0.018) favored longer relapse-free survival (RFS). CR after one course (HR=0.251, 95% CI 0.121-0.523, P< 0.001) and lower number of peripheral blood blast (HR=0.219, 95% CI 0.088-0.545, P=0.003) favored longer overall survival (OS) in AML with FLT3-ITD. Conclusion: FLT3-ITD positive AML with lower number of peripheral blood blast has a relative favorable prognosis. CR after one course and allotransplantation in CR1 are important strategy to improve prognosis in AML with FLT3-ITD.

Homoharringtonine (omacetaxine mepesuccinate) as an adjunct for FLT3-ITD acute myeloid leukemia

An in vitro drug-screening platform on patient samples was developed and validated to design personalized treatment for relapsed/refractory acute myeloid leukemia (AML). Unbiased clustering and correlation showed that homoharringtonine (HHT), also known as omacetaxine mepesuccinate, exhibited preferential antileukemia effect against AML carrying internal tandem duplication of fms-like tyrosine kinase 3 (FLT3-ITD). It worked synergistically with FLT3 inhibitors to suppress leukemia growth in vitro and in xenograft mouse models. Mechanistically, the effect was mediated by protein synthesis inhibition and reduction of short-lived proteins, including total and phosphorylated forms of FLT3 and its downstream signaling proteins. A phase 2 clinical trial of sorafenib and HHT combination treatment in FLT3-ITD AML patients resulted in complete remission (true or with insufficient hematological recovery) in 20 of 24 patients (83.3%), reduction of ITD allelic burden, and median leukemia-free and overall survivals of 12 and 33 weeks. The regimen has successfully bridged five patients to allogeneic hematopoietic stem cell transplantation and was well tolerated in patients unfit for conventional chemotherapy, including elderly and heavily pretreated patients. This study validated the principle and clinical relevance of in vitro drug testing and identified an improved treatment for FLT3-ITD AML. The results provided the foundation for phase 2/3 clinical trials to ascertain the clinical efficacy of FLT3 inhibitors and HHT in combination.

Discovery and Rational Design of Pteridin-7(8H)-one-Based Inhibitors Targeting FMS-like Tyrosine Kinase 3 (FLT3) and Its Mutants

FLT3 has been validated as a therapeutic target for the treatment of acute myeloid leukemia (AML). In this paper, we describe for the first time, pteridin-7(8H)-one as a scaffold for potent FLT3 inhibitors derived from structural optimizations on irreversible EGFR inhibitors. The representative inhibitor (31) demonstrates single-digit nanomolar inhibition against FLT3 and subnanomolar KD for drug-resistance FLT3 mutants. In profiling of the in vitro tumor cell lines, it shows good selectivity against AML cells harboring FLT3-ITD mutations over other leukemia and solid tumor cell lines. The mechanism of action study illustrates that pteridin-7(8H)-one derivatives suppress the phosphorylation of FLT3 and its downstream pathways, thereby inducing G0/G1 cell cycle arrest and apoptosis in AML cells. In in vivo studies, 31 significantly suppresses the tumor growth in MV4-11 xenograft model. Overall, we provide a structurally distinct chemical scaffold with which to develop FLT3 mutants-selective inhibitors for AML treatment.

Eradication of Acute Myeloid Leukemia with FLT3 Ligand-Targeted miR-150 Nanoparticles

Acute myeloid leukemia (AML) is a common and fatal form of hematopoietic malignancy. Overexpression and/or mutations of FLT3 have been shown to occur in the majority of cases of AML. Our analysis of a large-scale AML patient cohort (N = 562) indicates that FLT3 is particularly highly expressed in some subtypes of AML, such as AML with t(11q23)/MLL-rearrangements or FLT3-ITD. Such AML subtypes are known to be associated with unfavorable prognosis. To treat FLT3-overexpressing AML, we developed a novel targeted nanoparticle system: FLT3 ligand (FLT3L)-conjugated G7 poly(amidoamine) (PAMAM) nanosized dendriplex encapsulating miR-150, a pivotal tumor suppressor and negative regulator of FLT3 We show that the FLT3L-guided miR-150 nanoparticles selectively and efficiently target FLT3-overexpressing AML cells and significantly inhibit viability/growth and promote apoptosis of the AML cells. Our proof-of-concept animal model studies demonstrate that the FLT3L-guided miR-150 nanoparticles tend to concentrate in bone marrow, and significantly inhibit progression of FLT3-overexpressing AML in vivo, while exhibiting no obvious side effects on normal hematopoiesis. Collectively, we have developed a novel targeted therapeutic strategy, using FLT3L-guided miR-150-based nanoparticles, to treat FLT3-overexpressing AML with high efficacy and minimal side effects. Cancer Res; 76(15); 4470-80. ©2016 AACR.

Midostaurin: an emerging treatment for acute myeloid leukemia patients

Acute myeloid leukemia (AML) is a hematologic malignancy that carries a poor prognosis and has garnered few treatment advances in the last few decades. Mutation of the internal tandem duplication (ITD) region of fms-like tyrosine kinase (FLT3) is considered high risk for decreased response and overall survival. Midostaurin is a Type III receptor tyrosine kinase inhibitor found to inhibit FLT3 and other receptor tyrosine kinases, including platelet-derived growth factor receptors, cyclin-dependent kinase 1, src, c-kit, and vascular endothelial growth factor receptor. In preclinical studies, midostaurin exhibited broad-spectrum antitumor activity toward a wide range of tumor xenografts, as well as an FLT3-ITD-driven mouse model of myelodysplastic syndrome (MDS). Midostaurin is orally administered and generally well tolerated as a single agent; hematologic toxicity increases substantially when administered in combination with standard induction chemotherapy. Clinical trials primarily have focused on relapsed/refractory AML and MDS and included single- and combination-agent studies. Administration of midostaurin to relapsed/refractory MDS and AML patients confers a robust anti-blast response sufficient to bridge a minority of patients to transplant. In combination with histone deacetylase inhibitors, responses appear comparable to historic controls, while the addition of midostaurin to standard induction chemotherapy may prolong survival in FLT3-ITD mutant patients. The response of some wild-type (WT)-FLT3 patients to midostaurin therapy is consistent with midostaurin’s ability to inhibit WT-FLT3 in vitro, and also may reflect overexpression of WT-FLT3 in those patients and/or off-target effects such as inhibition of kinases other than FLT3. Midostaurin represents a well-tolerated, easily administered oral agent with the potential to bridge mutant and WT-FLT3 AML patients to transplant and possibly deepen response to induction chemotherapy. Ongoing studies are investigating midostaurin’s role in pretransplant induction and posttransplant consolidation therapy.

Molecular and Cellular Mechanisms of Myelodysplastic Syndrome: Implications on Targeted Therapy

Myelodysplastic syndrome (MDS) is a group of heterogeneous clonal hematopoietic stem cell disorders characterized by cytopenia, ineffective hematopoiesis, and progression to secondary acute myeloid leukemia in high-risk cases. Conventional prognostication relies on clinicopathological parameters supplemented by cytogenetic information. However, recent studies have shown that genetic aberrations also have critical impacts on treatment outcome. Moreover, these genetic alterations may themselves be a target for treatment. The mutation landscape in MDS is shaped by gene aberrations involved in DNA methylation (TET2, DNMT3A, IDH1/2), histone modification (ASXL1, EZH2), the RNA splicing machinery (SF3B1, SRSF2, ZRSR2, U2AF1/2), transcription (RUNX1, TP53, BCOR, PHF6, NCOR, CEBPA, GATA2), tyrosine kinase receptor signaling (JAK2, MPL, FLT3, GNAS, KIT), RAS pathways (KRAS, NRAS, CBL, NF1, PTPN11), DNA repair (ATM, BRCC3, DLRE1C, FANCL), and cohesion complexes (STAG2, CTCF, SMC1A, RAD21). A detailed understanding of the pathogenetic mechanisms leading to transformation is critical for designing single-agent or combinatorial approaches in target therapy of MDS.

Aberrant activation of the PI3K/mTOR pathway promotes resistance to sorafenib in AML

Therapy directed against oncogenic FLT3 has been shown to induce response in patients with acute myeloid leukemia (AML), but these responses are almost always transient. To address the mechanism of FLT3 inhibitor resistance, we generated two resistant AML cell lines by sustained treatment with the FLT3 inhibitor sorafenib. Parental cell lines carry the FLT3-ITD (tandem duplication) mutation and are highly responsive to FLT3 inhibitors, whereas resistant cell lines display resistance to multiple FLT3 inhibitors. Sanger sequencing and protein mass-spectrometry did not identify any acquired mutations in FLT3 in the resistant cells. Moreover, sorafenib treatment effectively blocked FLT3 activation in resistant cells, whereas it was unable to block colony formation or cell survival, suggesting that the resistant cells are no longer FLT3 dependent. Gene expression analysis of sensitive and resistant cell lines, as well as of blasts from patients with sorafenib-resistant AML, suggested an enrichment of the PI3K/mTOR pathway in the resistant phenotype, which was further supported by next-generation sequencing and phospho-specific-antibody array analysis. Furthermore, a selective PI3K/mTOR inhibitor, gedatolisib, efficiently blocked proliferation, colony and tumor formation, and induced apoptosis in resistant cell lines. Gedatolisib significantly extended survival of mice in a sorafenib-resistant AML patient-derived xenograft model. Taken together, our data suggest that aberrant activation of the PI3K/mTOR pathway in FLT3-ITD-dependent AML results in resistance to drugs targeting FLT3.

Molecularly targeted therapy in acute myeloid leukemia

Acute myeloid leukemia (AML) is molecularly heterogeneous. Formerly categorized cytogenetically and molecularly, AML may be classified by genomic and epigenomic analyses. These genetic lesions provide therapeutic targets. Genes targeted currently include mutated FLT3, NPM1 and KIT with drugs entering Phase III trials. Complete remission can be achieved in relapsed/refractory AML, albeit mostly transient. Mutated epigenetic modifiers, including DNMT3A, IDH1/2 and TET2, can be targeted by small molecule inhibitors, hypomethylating agents and histone deacetylase inhibitors. Other agents include cellular signaling pathway inhibitors and monoclonal antibodies against myeloid-associated antigens. Combinatorial strategies appear logical, mostly involving smaller molecular inhibitors partnering with hypomethylating agents. Currently limited to relapsed/refractory AML, targeted therapies are increasingly tested in frontline treatment with or without standard chemotherapy.

Clinical evaluation of panel testing by next-generation sequencing (NGS) for gene mutations in myeloid neoplasms

BACKGROUND

Genomic techniques in recent years have allowed the identification of many mutated genes important in the pathogenesis of acute myeloid leukemia (AML). Together with cytogenetic aberrations, these gene mutations are powerful prognostic markers in AML and can be used to guide patient management, for example selection of optimal post-remission therapy. The mutated genes also hold promise as therapeutic targets themselves. We evaluated the applicability of a gene panel for the detection of AML mutations in a diagnostic molecular pathology laboratory.

METHODS

Fifty patient samples comprising 46 AML and 4 other myeloid neoplasms were accrued for the study. They consisted of 19 males and 31 females at a median age of 60 years (range: 18-88 years). A total of 54 genes (full coding exons of 15 genes and exonic hotspots of 39 genes) were targeted by 568 amplicons that ranged from 225 to 275 bp. The combined coverage was 141 kb in sequence length. Amplicon libraries were prepared by TruSight myeloid sequencing panel (Illumina, CA) and paired-end sequencing runs were performed on a MiSeq (Illumina) genome sequencer. Sequences obtained were analyzed by in-house bioinformatics pipeline, namely BWA-MEM, Samtools, GATK, Pindel, Ensembl Variant Effect Predictor and a novel algorithm ITDseek.

RESULTS

The mean count of sequencing reads obtained per sample was 3.81 million and the mean sequencing depth was over 3000X. Seventy-seven mutations in 24 genes were detected in 37 of 50 samples (74 %). On average, 2 mutations (range 1-5) were detected per positive sample. TP53 gene mutations were found in 3 out of 4 patients with complex and unfavorable cytogenetics. Comparing NGS results with that of conventional molecular testing showed a concordance rate of 95.5 %. After further resolution and application of a novel bioinformatics algorithm ITDseek to aid the detection of FLT3 internal tandem duplication (ITD), the concordance rate was revised to 98.2 %.

CONCLUSIONS

Gene panel testing by NGS approach was applicable for sensitive and accurate detection of actionable AML gene mutations in the clinical laboratory to individualize patient management. A novel algorithm ITDseek was presented that improved the detection of FLT3-ITD of varying length, position and at low allelic burden.

Fishing the targets of myeloid malignancies in the era of next generation sequencing

Recent advent in next generation sequencing (NGS) and bioinformatics has generated an unprecedented amount of genetic information in myeloidmalignancies. This information may shed lights to the pathogenesis, diagnosis and prognostication of these diseases and provide potential targets for therapeutic intervention. However, the rapid emergence of genetic information will quickly outpace their functional validation by conventional laboratory platforms. Foundational knowledge about zebrafish hematopoiesis accumulated over the past two decades and novel genomeediting technologies and research strategies in thismodel organismhavemade it a unique and timely research tool for the study of human blood diseases. Recent studies modeling human myeloid malignancies in zebrafish have also highlighted the technical feasibility and clinical relevance of thesemodels. Careful validation of experimental protocols and standardization among laboratorieswill further enhance the application of zebrafish in the scientific communities and provide important insights to the personalized treatment ofmyeloid malignancies.

Identification of an orally available compound with potent and broad FLT3 inhibition activity

FLT3 internal tandem duplication (FLT3-ITD) is an activating mutation found in 20%-30% of patients with acute myeloid leukemia (AML), which makes FLT3 an attractive target for the treatment of AML. Although FLT3-mutant patients respond to current FLT3 inhibitors, relapse usually happens due to the acquisition of resistant secondary mutations at the FLT3 catalytic domain, which is mainly on D835. In the search for compounds with broad FLT3 inhibition activities, we screened a kinase inhibitor library by using our unique FLT3 substrate and identified JAK3 inhibitor VI (designated JI6 hereafter) as a novel FLT3 inhibitor, which selectively targets FLT3 D835 mutants as well as FLT3-ITD. JI6 effectively inhibited FLT3-ITD-containing MV4-11 cells and HCD-57 cells transformed with FLT3-ITD and D835 mutants. Furthermore, administration of JI6 effectively targeted FLT3 signaling in vivo and suppressed the myeloproliferative phenotypes in FLT3-ITD knock-in mice and significantly prolonged the survival of immunodeficient mice implanted with the transformed HCD-57 cells. Therefore, JI6 is a promising candidate for development of next generation anti-AML drugs.

Pim kinases modulate resistance to FLT3 tyrosine kinase inhibitors in FLT3-ITD acute myeloid leukemia

Fms-like tyrosine kinase 3 internal tandem duplication (FLT3-ITD) is frequently detected in acute myeloid leukemia (AML) patients and is associated with a dismal long-term prognosis. FLT3 tyrosine kinase inhibitors provide short-term disease control, but relapse invariably occurs within months. Pim protein kinases are oncogenic FLT3-ITD targets expressed in AML cells. We show that increased Pim kinase expression is found in relapse samples from AML patients treated with FLT3 inhibitors. Ectopic Pim-2 expression induces resistance to FLT3 inhibition in both FLT3-ITD-induced myeloproliferative neoplasm and AML models in mice. Strikingly, we found that Pim kinases govern FLT3-ITD signaling and that their pharmacological or genetic inhibition restores cell sensitivity to FLT3 inhibitors. Finally, dual inhibition of FLT3 and Pim kinases eradicates FLT3-ITD(+) cells including primary AML cells. Concomitant Pim and FLT3 inhibition represents a promising new avenue for AML therapy.

Class III Receptor Tyrosine Kinases in Acute Leukemia - Biological Functions and Modern Laboratory Analysis

Acute myeloid leukemia (AML) is a complex disease caused by deregulation of multiple signaling pathways. Mutations in class III receptor tyrosine kinases (RTKs) have been implicated in alteration of cell signals concerning the growth and differentiation of leukemic cells. Point mutations, insertions, or deletions of RTKs as well as chromosomal translocations induce constitutive activation of the receptor, leading to uncontrolled proliferation of undifferentiated myeloid blasts. Aberrations can occur in all domains of RTKs causing either the ligand-independent activation or mimicking the activated conformation. The World Health Organization recommended including RTK mutations in the AML classification since their detection in routine laboratory diagnostics is a major factor for prognostic stratification of patients. Polymerase chain reaction (PCR)-based methods are well-validated for the detection of fms-related tyrosine kinase 3 (FLT3) mutations and can easily be applied for other RTKs. However, when methodological limitations are reached, accessory techniques can be applied. For a higher resolution and more quantitative approach compared to agarose gel electrophoresis, PCR fragments can be separated by capillary electrophoresis. Furthermore, high-resolution melting and denaturing high-pressure liquid chromatography are reliable presequencing screening methods that reduce the sample amount for Sanger sequencing. Because traditional DNA sequencing is time-consuming, next-generation sequencing (NGS) is an innovative modern possibility to analyze a high amount of samples simultaneously in a short period of time. At present, standardized procedures for NGS are not established, but when this barrier is resolved, it will provide a new platform for rapid and reliable laboratory diagnostic of RTK mutations in patients with AML. In this article, the biological and physiological role of RTK mutations in AML as well as possible laboratory methods for their detection will be reviewed.

Homology modeling of DFG-in FMS-like tyrosine kinase 3 (FLT3) and structure-based virtual screening for inhibitor identification

The inhibition of FMS-like tyrosine kinase 3 (FLT3) activity using small-molecule inhibitors has emerged as a target-based alternative to traditional chemotherapy for the treatment of acute myeloid leukemia (AML). In this study, we report the use of structure-based virtual screening (SBVS), a computer-aided drug design technique for the identification of new chemotypes for FLT3 inhibition. For this purpose, homology modeling (HM) of the DFG-in FLT3 structure was carried using two template structures, including PDB ID: 1RJB (DFG-out FLT3 kinase domain) and PDB ID: 3LCD (DFG-in CSF-1 kinase domain). The modeled structure was able to correctly identify known DFG-in (SU11248, CEP-701, and PKC-412) and DFG-out (sorafenib, ABT-869 and AC220) FLT3 inhibitors, in docking studies. The modeled structure was then used to carry out SBVS of an HTS library of 125,000 compounds. The top scoring 97 compounds were tested for FLT3 kinase inhibition, and two hits (BPR056, IC₅₀ = 2.3 and BPR080, IC₅₀ = 10.7 μM) were identified. Molecular dynamics simulation and density functional theory calculation suggest that BPR056 (MW: 325.32; cLogP: 2.48) interacted with FLT3 in a stable manner and could be chemically optimized to realize a drug-like lead in the future.

Perspectives for therapeutic targeting of gene mutations in acute myeloid leukaemia with normal cytogenetics

The acute myeloid leukaemia (AML) genome contains more than 20 driver recurrent mutations. Here, we review the potential for therapeutic targeting of the most common mutations associated with normal cytogenetics AML, focusing on those affecting the FLT3, NPM1 and epigenetic modifier genes (DNMT3A, IDH1/2, TET2). As compared to early compounds, second generation FLT3 inhibitors are more specific and have better pharmacokinetics. They also show higher anti-leukaemic activity, leading to about 50% of composite complete remissions in refractory/relapsed FLT3-internal tandem duplication-mutated AML. However, rapid relapses invariably occur due to various mechanisms of resistance to FLT3 inhibitors. This issue and the best way for using FLT3 inhibitors in combination with other therapeutic modalities are discussed. Potential approaches for therapeutic targeting of NPM1-mutated AML include: (i) reverting the aberrant nuclear export of NPM1 mutant using exportin-1 inhibitors; (ii) disruption of the nucleolus with drugs blocking the oligomerization of wild-type nucleophosmin or inducing nucleolar stress; and (iii) immunotherapeutic targeting of highly expressed CD33 and IL3RA (CD123) antigens. Finally, we discuss the role of demethylating agents (decitabine and azacitidine) and IDH1/2 inhibitors in the treatment of AML patients carrying mutations of genes (DNMT3A, IDH1/2 and TET2) involved in the epigenetic regulation of transcription.

Targeting of FLT3-ITD kinase contributes to high selectivity of imidazoacridinone C-1311 against FLT3-activated leukemia cells

Drugs targeting receptor tyrosine kinase FLT3 are of particular interest since activating FLT3-internal tandem duplication (ITD) mutations abundantly occur in fatal acute myeloid leukemias (AMLs). Imidazoacridinone C-1311, a DNA-reactive inhibitor of topoisomerase II, has been previously shown to be a potent and selective inhibitor of recombinant FLT3. Here, we expand those findings by studying its effect on leukemia cells with wild-type FLT3, FLT3-ITD mutant and no FLT3 receptor. While brief C-1311 exposure blocked wild-type and FLT3-ITD activity, profound and sustained inhibition was achieved only for FLT3-ITD mutants. C-1311 inhibited FLT3 downstream pathways (MAPK and AKT) independent of FLT3 status, yet translation to decreased viability was significant in FLT3-ITD cells. RNA interference against FLT3-ITD reduced cytotoxic effect and apoptosis induced by C-1311, indicating selective inhibition of FLT3-ITD crucial for high efficacy of drug against activated leukemia cells. Cellular responses in treated FLT3-ITD mutants included G1 and G2/M phase arrest, moderate inhibition of Bcl-2, caspase-3 activation, PARP cleavage, and depolarization of mitochondria. Consistent with selective decrease in FLT3-ITD activity, C-1311 remarkably reduced antiapoptotic survivin mRNA and protein expression, correlating well with enhanced apoptosis of FLT3-ITD cells. No survivin decrease and respectively lower level of apoptosis was found in wild-type and null-FLT3 cells. Combination of C-1311 with cytarabine or doxorubicin again showed distinct synergistic activity in FLT3-ITD-positive cells. The ability of C-1311 to selectively target constitutively active FLT3, suggests a favorable therapeutic index for AML carrying FLT3-ITD mutations. Thus further preclinical and clinical studies addressing its potency against FLT3-ITD kinase is well justified.

Oncogene addiction: pathways of therapeutic response, resistance, and road maps toward a cure

A key goal of cancer therapeutics is to selectively target the genetic lesions that initiate and maintain cancer cell proliferation and survival. While most cancers harbor multiple oncogenic mutations, a wealth of preclinical and clinical data supports that many cancers are sensitive to inhibition of single oncogenes, a concept referred to as 'oncogene addiction'. Herein, we describe the clinical evidence supporting oncogene addiction and discuss common mechanistic themes emerging from the response and acquired resistance to oncogene-targeted therapies. Finally, we suggest several opportunities toward exploiting oncogene addiction to achieve curative cancer therapies.

Maintenance of leukemia-initiating cells is regulated by the CDK inhibitor Inca1

Functional differences between healthy progenitor and cancer initiating cells may provide unique opportunities for targeted therapy approaches. Hematopoietic stem cells are tightly controlled by a network of CDK inhibitors that govern proliferation and prevent stem cell exhaustion. Loss of Inca1 led to an increased number of short-term hematopoietic stem cells in older mice, but Inca1 seems largely dispensable for normal hematopoiesis. On the other hand, Inca1-deficiency enhanced cell cycling upon cytotoxic stress and accelerated bone marrow exhaustion. Moreover, AML1-ETO9a-induced proliferation was not sustained in Inca1-deficient cells in vivo. As a consequence, leukemia induction and leukemia maintenance were severely impaired in Inca1^−/− bone marrow cells. The re-initiation of leukemia was also significantly inhibited in absence of Inca1^−/− in MLL—AF9- and c-myc/BCL2-positive leukemia mouse models. These findings indicate distinct functional properties of Inca1 in normal hematopoietic cells compared to leukemia initiating cells. Such functional differences might be used to design specific therapy approaches in leukemia.

Tracing dynamics and clonal heterogeneity of Cbx7-induced leukemic stem cells by cellular barcoding

Accurate monitoring of tumor dynamics and leukemic stem cell (LSC) heterogeneity is important for the development of personalized cancer therapies. In this study, we experimentally induced distinct types of leukemia in mice by enforced expression of Cbx7. Simultaneous cellular barcoding allowed for thorough analysis of leukemias at the clonal level and revealed high and unpredictable tumor complexity. Multiple LSC clones with distinct leukemic properties coexisted. Some of these clones remained dormant but bore leukemic potential, as they progressed to full-blown leukemia after challenge. LSC clones could retain multilineage differentiation capacities, where one clone induced phenotypically distinct leukemias. Beyond a detailed insight into CBX7-driven leukemic biology, our model is of general relevance for the understanding of tumor dynamics and clonal evolution.

Characterization of gene mutations and copy number changes in acute myeloid leukemia using a rapid target enrichment protocol

Prognostic stratification is critical for making therapeutic decisions and maximizing survival of patients with acute myeloid leukemia. Advances in the genomics of acute myeloid leukemia have identified several recurrent gene mutations whose prognostic impact is being deciphered. We used HaloPlex target enrichment and Illumina-based next generation sequencing to study 24 recurrently mutated genes in 42 samples of acute myeloid leukemia with a normal karyotype. Read depth varied between and within genes for the same sample, but was predictable and highly consistent across samples. Consequently, we were able to detect copy number changes, such as an interstitial deletion of BCOR, three MLL partial tandem duplications, and a novel KRAS amplification. With regards to coding mutations, we identified likely oncogenic variants in 41 of 42 samples. NPM1 mutations were the most frequent, followed by FLT3, DNMT3A and TET2. NPM1 and FLT3 indels were reported with good efficiency. We also showed that DNMT3A mutations can persist post-chemotherapy and in 2 cases studied at diagnosis and relapse, we were able to delineate the dynamics of tumor evolution and give insights into order of acquisition of variants. HaloPlex is a quick and reliable target enrichment method that can aid diagnosis and prognostic stratification of acute myeloid leukemia patients.

Proton channels and exchangers in cancer

Although cancer is characterized by an intratumoral genetic heterogeneity, a totally deranged pH control is a common feature of most cancer histotypes. Major determinants of aberrant pH gradient in cancer are proton exchangers and transporters, including V-ATPase, Na+/H+ exchanger (NHE), monocarboxylate transporters (MCTs) and carbonic anhydrases (CAs). Thanks to the activity of these proton transporters and exchangers, cancer becomes isolated and/or protected not only from the body reaction against the growing tumor, but also from the vast majority of drugs that when protonated into the acidic tumor microenvironment do not enter into cancer cells. Proton transporters and exchangers represent a key feature tumor cells use to survive in the very hostile microenvironmental conditions that they create and maintain. Detoxifying mechanisms may thus represent both a key survival option and a selection outcome for cells that behave as unicellular microorganisms rather than belonging to an organ, compartment or body. It is, in fact, typical of malignant tumors that, after a clinically measurable yet transient initial response to a therapy, resistant tumor clones emerge and proliferate, thus bursting a more malignant behavior and rapid tumor progression. This review critically presents the background of a novel and efficient approach that aims to fight cancer through blocking or inhibiting well characterized proton exchangers and transporters active in human cancer cells. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Designed Macrocyclic Kinase Inhibitors

Cancer continues to present as an increasing and serious global unmet medical need in today's aging population.1 Macrocyclic kinase inhibitors have reached advanced clinical testing and are making an impact in oncologic conditions including myelofibrosis, lymphomas and leukemias. Rheumatoid arthritis (RA) is also beginning to be impacted with the first macrocycle having entered Phase I clinical evaluation in healthy volunteers. Increasing reports of innovative macrocycles in preclinical research are appearing in the literature. Desirable, selective, multi-kinase inhibitory profiles against specific kinases known to be abrogated in cancer, RA, and other diseases have been achieved in a first generation series of clinical stage compact small molecule macrocyclic kinase inhibitors. Herein we discuss their design, synthesis, structure activity relationships and assessment of the latest clinical data in a range of oncologic conditions. Macrocyclic kinase inhibitors have the potential to offer new hope to patients and their families.

Genetic mutations in acute myeloid leukemia that influence clinical decisions

PURPOSE OF REVIEW

A plethora of studies over the past two decades have identified many genes that are recurrently mutated in acute myeloid leukemia (AML). Although great advances have been made in understanding the role of these mutated genes in AML disease pathogenesis, to date relatively few have been demonstrated to have direct clinical relevance.

RECENT FINDINGS

Genomic techniques have allowed for the identification of many mutated genes that appear to drive disease pathogenesis and prognosis in AML. Integrated analyses examining the co-occurrence of these genes in well annotated AML patient cohorts has helped to significantly refine prognostic models, allowing for a more nuanced selection of patients for optimal postremission therapies. Furthermore, there are emerging data that gene mutations may be useful to select patients for optimal doses and/or modalities of upfront AML therapy. Finally, mutated genes themselves hold promise as therapeutic targets, as supported by strong preclinical studies.

SUMMARY

Recent advances in our knowledge of the molecular genetics of AML have significantly improved our tools for clinical decision-making and promise to identify new therapies for patients.

Use of signaling pathways as therapeutic targets for blood cancer

SUMMARY 

Until recently, the treatment of blood cancers has rested exclusively on chemotherapy, radiation and, in select cases, stem cell transplantation, giving rise to frequent and sometimes life-threatening side effects. The past 10 years have witnessed dramatic clinical advances due to the development of novel therapies, hereafter referred to as targeted therapies, which specifically inhibit molecules that are essential to the pathophysiology of individual malignancies. In this article, we will discuss the general concept of targeting signaling pathways in cancers and the limitations of this strategy, with a particular focus on the emergence of resistant cancer clones under the selective pressures exerted by targeted therapies. Finally, we will examine a number of targeted therapies with immediate application in contemporary clinical practice.

FLT3 Internal Tandem Duplication and D835 Mutations in Patients with Acute Lymphoblastic Leukemia and its Clinical Significance

The fms-like tyrosine kinase 3 (FLT3) gene is a member of the class III receptor tyrosine kinase family. Mutations of FLT3 were first described in 1997 and account for the most frequent molecular mutations in acute myeloid leukemia.

Currently, there is no published data on FLT3 mutations in Saudi acute lymphoblastic leukemia (ALL) patients.

In this retrospective study, we have examined a cohort of 77 ALL patients to determine the prevalence of FLT3 mutations and the possible prognostic relevance of these mutations in ALL patients. Correlations to other biologic factors such as karyotype, molecular mutations, and leukocyte count were also considered.

FLT3 internal tandem duplication (ITD) mutations and point mutation in tyrosine kinase domain (D835) were analyzed in ALL patients, at diagnosis, by polymerase chain reaction (PCR).

Two cases (2.6%, 2/77) were positive for FLT3 mutations; one was found to have FLT3/ITD and the other FLT3/D835.

Our findings suggest that FLT3 mutations are not common in Saudi ALL and do not affect clinical outcome.

Functions of flt3 in zebrafish hematopoiesis and its relevance to human acute myeloid leukemia

FMS-like tyrosine kinase 3 (FLT3) is expressed in human hematopoietic stem and progenitor cells (HSPCs) but its role during embryogenesis is unclear. In acute myeloid leukemia (AML), internal tandem duplication (ITD) of FLT3 at the juxtamembrane (JMD) and tyrosine kinase (TKD) domains (FLT3-ITD(+)) occurs in 30% of patients and is associated with inferior clinical prognosis. TKD mutations (FLT3-TKD(+)) occur in 5% of cases. We made use of zebrafish to examine the role of flt3 in developmental hematopoiesis and model human FLT3-ITD(+) and FLT3-TKD(+) AML. Zebrafish flt3 JMD and TKD were remarkably similar to their mammalian orthologs. Morpholino knockdown significantly reduced the expression of l-plastin (pan-leukocyte), csf1r, and mpeg1 (macrophage) as well as that of c-myb (definitive HSPCs), lck, and rag1 (T-lymphocyte). Expressing human FLT3-ITD in zebrafish embryos resulted in expansion and clustering of myeloid cells (pu.1(+), mpo(+), and cebpα(+)) which were ameliorated by AC220 and associated with stat5, erk1/2, and akt phosphorylation. Human FLT3-TKD (D835Y) induced significant, albeit modest, myeloid expansion resistant to AC220. This study provides novel insight into the role of flt3 during hematopoiesis and establishes a zebrafish model of FLT3-ITD(+) and FLT3-TKD(+) AML that may facilitate high-throughput screening of novel and personalized agents.

A novel tescalcin-sodium/hydrogen exchange axis underlying sorafenib resistance in FLT3-ITD+ AML

A novel TESC-NEH1 pathway is involved in FLT3-ITD+ AML pathogenesis. Inhibition of NHE1 overcomes sorafenib resistance in FLT3-ITD+AML.

Proteinase-Activated Receptor 1 (PAR1) regulates leukemic stem cell functions

External signals that are mediated by specific receptors determine stem cell fate. The thrombin receptor PAR1 plays an important role in haemostasis, thrombosis and vascular biology, but also in tumor biology and angiogenesis. Its expression and function in hematopoietic stem cells is largely unknown. Here, we analyzed expression and function of PAR1 in primary hematopoietic cells and their leukemic counterparts. AML patients' blast cells expressed much lower levels of PAR1 mRNA and protein than CD34⁺ progenitor cells. Constitutive Par1-deficiency in adult mice did not affect engraftment or stem cell potential of hematopoietic cells. To model an AML with Par1-deficiency, we retrovirally introduced the oncogene MLL-AF9 in wild type and Par1^−/− hematopoietic progenitor cells. Par1-deficiency did not alter initial leukemia development. However, the loss of Par1 enhanced leukemic stem cell function in vitro and in vivo. Re-expression of PAR1 in Par1^−/− leukemic stem cells delayed leukemogenesis in vivo. These data indicate that Par1 contributes to leukemic stem cell maintenance.

Feedbacks and adaptive capabilities of the PI3K/Akt/mTOR axis in acute myeloid leukemia revealed by pathway selective inhibition and phosphoproteome analysis

Acute myeloid leukemia (AML) primary cells express high levels of phosphorylated Akt, a master regulator of cellular functions regarded as a promising drug target. By means of reverse phase protein arrays, we examined the response of 80 samples of primary cells from AML patients to selective inhibitors of the phosphatidylinositol 3 kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) axis. We confirm that >60% of the samples analyzed are characterized by high pathway phosphorylation. Unexpectedly, however, we show here that targeting Akt and mTOR with the specific inhibitors Akti 1/2 and Torin1, alone or in combination, result in paradoxical Akt phosphorylation and activation of downstream signaling in 70% of the samples. Indeed, we demonstrate that cropping Akt or mTOR activity can stabilize the Akt/mTOR downstream effectors Forkhead box O and insulin receptor substrate-1, which in turn potentiate signaling through upregulation of the expression/phosphorylation of selected growth factor receptor tyrosine kinases (RTKs). Activation of RTKs in turn reactivates PI3K and downstream signaling, thus overruling the action of the drugs. We finally demonstrate that dual inhibition of Akt and RTKs displays strong synergistic cytotoxic effects in AML cells and downmodulates Akt signaling to a much greater extent than either drug alone, and should therefore be explored in AML clinical setting.

Over-expression of FoxM1 is associated with adverse prognosis and FLT3-ITD in acute myeloid leukemia

Forkhead box M1 (FoxM1) drives cell cycle progression and the prevention of growth arrest and is over-expressed in many human malignancies. However, the characteristics of FoxM1 in acute myeloid leukemia (AML) are not clearly understood. We investigated the expression level of FoxM1 and analyzed the correlation of FoxM1 expression with AML patient characteristics and prognoses. Changes in FoxM1 expression were detected after MV4-11 cells, which have an internal tandem duplication (ITD) of the fms-like tyrosine kinase 3 gene (FLT3-ITD), and control THP1 cells (encoding wild-type FLT3) were treated with the FLT3 receptor tyrosine kinase inhibitor AC220 (quizartinib) or FLT3 ligand (FL). Finally, we determined the apoptosis rates after the addition of the FoxM1 inhibitor thiostrepton (TST) to AML cells with or without FLT3-ITD. The expression of FoxM1 in AML patients was correlated with the presence of FLT3-ITD, genetic groups, and possibly overall survival. Inhibition of FLT3-ITD by AC220 down-regulated FoxM1 expression in MV4-11 cells, and stimulation of FLT3 by FL up-regulated FoxM1 expression in MV4-11 and THP1 cells. TST induced the apoptosis of MV4-11 and THP1 cells in a dose-dependent manner. Thus, FoxM1 is a potential prognostic marker and a promising therapeutic target in AML.

Frequency and Prognostic Relevance of FLT3 Mutations in Saudi Acute Myeloid Leukemia Patients

The Fms-like tyrosine kinase-3 (FLT3) is a receptor tyrosine kinase that plays a key role in cell survival, proliferation, and differentiation of hematopoietic stem cells. Mutations of FLT3 were first described in 1997 and account for the most frequent molecular mutations in acute myeloid leukemia (AML). AML patients with FLT3 internal tandem duplication (ITD) mutations have poor cure rates the prognostic significance of point mutations; tyrosine kinase domain (TKD) is still unclear. We analyzed the frequency of FLT3 mutations (ITD and D835) in patients with AML at diagnosis; no sufficient data currently exist regarding FLT3 mutations in Saudi AML patients. This study was aimed at evaluating the frequency of FLT3 mutations in patients with AML and its significance for prognosis. The frequency of FLT3 mutations in our study (18.56%) was lower than many of the reported studies, FLT3-ITD mutations were observed in 14.4%, and FLT3-TKD in 4.1%, of 97 newly diagnosed AML patients (82 adult and 15 pediatric). Our data show significant increase of FLT3 mutations in male more than female (13 male, 5 female). Our results support the view that FLT3-ITD mutation has strong prognostic factor in AML patients and is associated with high rate of relapse, and high leucocytes and blast count at diagnosis and relapse.

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.

Incidence of internal tandem duplications and D835 mutations of FLT3 gene in chronic myeloid leukemia patients from Southern India

OBJECTIVE

To screen two important FLT3 mutations (internal tandem duplication (ITD) and D835 point mutations) in chronic myeloid leukemia (CML) patients from Southern India and report their incidence.

METHODS

Screened 350 CML patients and 350 controls for the two FLT3/mutations through polymerase chain reaction and restriction fragment length polymorphism methods.

RESULTS

ITDs were detected in 12 of the 350 CML patients (3.4%) and D835 mutations in only four cases (1.14%), relatively low in frequency as compared to those reported earlier from non-Indian populations. None of the cases showed simultaneous occurence of both ITD and D835 mutations.

DISCUSSION

These FLT3 mutations seem to be very rare in CML, and it is possible that these could be found only in a subset of patients who are in the progressive stage and/or with varied drug response. Prospective studies are needed to confirm the role of FLT3 mutations in CML pathogenesis, which may help devising therapeutic interventions.

CSF1R mutations in hereditary diffuse leukoencephalopathy with spheroids are loss of function

Hereditary diffuse leukoencephalopathy with spheroids (HDLS) in humans is a rare autosomal dominant disease characterized by giant neuroaxonal swellings (spheroids) within the CNS white matter. Symptoms are variable and can include personality and behavioural changes. Patients with this disease have mutations in the protein kinase domain of the colony-stimulating factor 1 receptor (CSF1R) which is a tyrosine kinase receptor essential for microglia development. We investigated the effects of these mutations on Csf1r signalling using a factor dependent cell line. Corresponding mutant forms of murine Csf1r were expressed on the cell surface at normal levels, and bound CSF1, but were not able to sustain cell proliferation. Since Csf1r signaling requires receptor dimerization initiated by CSF1 binding, the data suggest a mechanism for phenotypic dominance of the mutant allele in HDLS.

Development and validation of risk models and molecular diagnostics to permit personalized management of cancer

Despite the advances made in cancer management over the past few decades, improvements in cancer diagnosis and prognosis are still poor, highlighting the need for individualized strategies. Toward this goal, risk prediction models and molecular diagnostic tools have been developed, tailoring each step of risk assessment from diagnosis to treatment and clinical outcomes based on the individual's clinical, epidemiological, and molecular profiles. These approaches hold increasing promise for delivering a new paradigm to maximize the efficiency of cancer surveillance and efficacy of treatment. However, they require stringent study design, methodology development, comprehensive assessment of biomarkers and risk factors, and extensive validation to ensure their overall usefulness for clinical translation. In the current study, the authors conducted a systematic review using breast cancer as an example and provide general guidelines for risk prediction models and molecular diagnostic tools, including development, assessment, and validation.

Cancer-associated IDH2 mutants drive an acute myeloid leukemia that is susceptible to Brd4 inhibition

Somatic mutations in the isocitrate dehydrogenase (IDH) genes IDH1 and IDH2 occur frequently in acute myeloid leukemia (AML) and other cancers. These genes encode neomorphic proteins that produce the presumed oncometabolite 2-hydroxyglutarate (2-HG). Despite the prospect of treating AML and other cancers by targeting IDH mutant proteins, it remains unclear how these mutants affect tumor development and maintenance in vivo, and no cancer models exist to study the action of IDH2 mutants in vivo. We show that IDH2 mutants can cooperate with oncogenic Flt3 or Nras alleles to drive leukemia in mice by impairing the differentiation of cells of the myeloid lineage. Pharmacologic or genetic inhibition of IDH2 triggers the differentiation and death of AML cells, albeit only with prolonged IDH2 inhibition. In contrast, inhibition of the bromodomain-containing protein Brd4 triggers rapid differentiation and death of IDH2 mutant AML. Our results establish a critical role for mutant IDH2 in leukemogenesis and tumor maintenance and identify an IDH-independent strategy to target these cancers therapeutically.

SU11652 Inhibits tyrosine kinase activity of FLT3 and growth of MV-4-11 cells

Background

FLT3-ITD and FLT3-TKD mutations are frequently found in acute myeloid leukemia (AML). This makes tyrosine kinase FLT3 a highly attractive target for therapeutic drug development. However, effective drugs have not yet emerged. This study is intended to identify and to characterize new FLT3 inhibitors.

Methods

By using the protein substrate GST-FLT3S to analyze kinase activity of recombinant proteins carrying the catalytic domain of wild type and mutant forms of FLT3, we screened a chemical library containing 80 known protein kinase inhibitors. We identified SU11652 as a potent FLT3 inhibitor and further employed FLT3-ITD-positive MV- 4–11 cells to study its effects on cell growth, apoptosis, cell cycles, and cell signaling.

Results

SU11652 strongly inhibited the activity of wild type, D835Y, and D835H mutant forms of FLT3 with IC₅₀ values of 1.5, 16, and 32 nM, respectively. It effectively blocked the growth of FLT3-ITD -positive MV-4-11 cells at nanomolar concentrations but exhibited much less effects on several other cells which do not carry mutations of FLT3. SU11652 inhibited growth of MV-4-11 cells by inducing apoptosis, causing cell cycle arrest, and blocking activation of the ERK, Akt, and STAT signaling pathways.

Conclusion

SU11652 is a potent FLT3 inhibitor which selectively targets FLT3-ITD-positive cells. It should serve as a good candidate for development of therapeutic drugs to treat AML.