Identification of emerging FLT3 ITD‐positive clones during clinical remission and kinetics of disease relapse in acute myeloid leukaemia with mutated nucleophosmin

FLT3 internal tandem duplication (ITD) mutations are frequently detected at diagnosis in cytogenetically normal acute myeloid leukaemia (CN‐AML) and predict unfavourable outcome. FLT3 ITD is an unstable aberration and may be lost or acquired at relapse. Recent whole genome sequencing studies have suggested that FLT3 ITD⁺ve AML relapse may evolve from small subclones undetectable at diagnosis by routine polymerase chain reaction (PCR). We developed a patient‐specific real‐time quantitative‐PCR (RQ‐PCR) to implement FLT3 ITD detection in six AML patients whose blasts carried wild‐type FLT3 at diagnosis and who relapsed with FLT3 ITD by routine PCR. Patient‐specific forward primers were designed after cloning and sequencing the FLT3 ITD in each case. The assay allowed retrospective detection of FLT3 ITD in diagnostic samples of 4/6 cases and to establish the kinetics of clonal evolution preceding relapse. After conventional chemotherapy, all patients had early relapse despite having been classified as NPM1⁺ve/FLT3 ITD^?ve at presentation, with shorter remissions being observed in four patients re‐classified as FLT3 ITD⁺ve by the new assay. Notably, FLT3 ITD clone became detectable by conventional PCR in three patients tested during remission after initial treatment. Our data underscore the need of identifying low FLT3 ITD levels, which are probably associated with relapse in otherwise good prognosis CN‐AML.     

Bone marrow stroma‐mediated resistance to FLT3 inhibitors in FLT3‐ITD AML is mediated by persistent activation of extracellular regulated kinase

A consistent pattern of response has been observed when FMS‐like tyrosine kinase 3 (FLT3) tyrosine kinase inhibitors (TKIs) have been used as monotherapy to treat patients with relapsed or refractory FLT3‐ internal tandem duplication (ITD) acute myeloid leukaemia (AML). Circulating blasts are cleared from the peripheral blood, while bone marrow blasts are either unaffected or are cleared from the marrow at a much slower rate. We used an in vitro model of FLT3‐ITD AML blasts co‐cultured with normal human bone marrow stromal cells to investigate the basis for this dichotomous response pattern to FLT3 inhibitors. We have found that in blasts on stroma, potent FLT3 inhibition predominantly results in cell cycle arrest rather than apoptosis. The anti‐apoptotic effect is mediated through a combination of direct cell‐cell contact and soluble factors. The addition of exogenous FLT3 ligand (FL) augments the protection, primarily by shifting the 50% inhibitory concentration for FLT3 inhibition upwards. Cytokine‐activated extracellular regulated kinase (ERK), rather than STAT5, appears to be the most important downstream signalling protein mediating the protective effect, and inhibition of MEK significantly abrogates stromal‐mediated resistance. These findings explain the phenomenon of peripheral blood versus bone marrow blast responses and suggest that the combination of potent FLT3 inhibition and MEK inhibition is a promising strategy for the treatment of FLT3‐ITD AML.     

The expression of P-glycoprotein in AML cells with FLT3 internal tandem duplications is associated with reduced apoptosis in response to FLT3 inhibitors

P-glycoprotein (pgp), a membrane efflux pump, is recognized to have an anti-apoptotic function. Internal tandem duplications (ITDs) of the Fms-like tyrosine kinase 3 (FLT3) receptor are the most common mutations in acute myeloid leukaemia (AML). Both ITDs and pgp positivity confer an adverse clinical prognosis. FLT3 inhibitors induce variable apoptosis in cell lines transfected with FLT3 ITDs. We studied the effect of herbimycin A, AG1296 and PKC412 on primary AML blasts. All compounds showed significantly higher cell kill after 48-h incubation in samples with an ITD compared with wild type (Herbimicin P < 0.001; AG1296 P = 0.001, PKC412, P = 0.002). Pgp-positive samples were significantly less sensitive to herbimycin and AG1296 than pgp-negative samples, although neither molecule inhibited the efflux function of pgp. The concurrent incubation with the pgp inhibitor PSC833 resulted in an enhanced cell kill in 4/5 ITD pgp-positive samples versus two of nine ITD pgp-negative samples. PKC412 inhibited pgp function and induced cell death in FLT3 ITD/pgp-positive samples. We conclude that AML samples with a FLT3 ITD are more susceptible to these inhibitors than wild-type samples. However, the expression of pgp in cells with FLT3 ITDs can reduce their sensitivity to FLT3 inhibitors and therefore pgp expression should be assessed in clinical trials of FLT3 inhibitors.     

A FLT3 tyrosine kinase inhibitor is selectively cytotoxic to acute myeloid leukemia blasts harboring FLT3 internal tandem duplication mutations

Internal tandem duplication (ITD) mutations of the receptor tyrosine kinase FLT3 have been found in 20% to 30% of patients with acute myeloid leukemia (AML). These mutations constitutively activate the receptor and appear to be associated with a poor prognosis. Recent evidence that this constitutive activation is leukemogenic renders this receptor a potential target for specific therapy. In this study, dose-response cytotoxic assays were performed with AG1295, a tyrosine kinase inhibitor active against FLT3, on primary blasts from patients with AML. For each patient sample, the degree of cytotoxicity induced by AG1295 was compared to the response to cytosine arabinoside (Ara C) and correlated with the presence or absence of a FLT3/ITD mutation. AG1295 was specifically cytotoxic to AML blasts harboring FLT3/ITD mutations. The results suggest that these mutations contribute to the leukemic process and that the FLT3 receptor represents a therapeutic target in AML. (Blood. 2001;98:885-887)     

The impact of FLT3 internal tandem duplication mutant level, number, size, and interaction with NPM1 mutations in a large cohort of young adult patients with acute myeloid leukemia

An internal tandem duplication in the fms-like tyrosine kinase 3 gene (FLT3/ITD) is associated with poor prognosis in acute myeloid leukemia (AML), but the impact of mutant level, size, and interaction with nucleophosmin 1 (NPM1) mutations remains controversial. We evaluated these characteristics in a large cohort of young adult AML patients. There was a highly significant trend for worsening in relapse risk (RR) and overall survival (OS) with increasing FLT3/ITD mutant level (P < .001 for both), and even in the low level mutant group (1%-24% of total FLT3 alleles), RR was significantly worse than in the FLT3 wild-type (WT) group (P < .001). In multivariate analysis, mutant level was the most powerful prognostic factor for RR. Mutant size and number had no significant impact on outcome. The beneficial impact of an NPM1 mutation on RR and OS was seen in FLT3/ITD(+) as well as FLT3/WT patients; both markers were highly significant independent predictors of outcome (P < .001). Stratification using both markers identified 3 prognostic groups: good (FLT3/ITD(-)NPM1(+)), intermediate (FLT3/ITD(-)NPM1(-) or FLT3/ITD(+)NPM1(+)), and poor (FLT3/ITD(+)NPM1(-)). Patients with high FLT3/ITD mutant level (greater than 50%) or FLT3/ITD(+) in the absence of an NPM1 mutation may be good candidates for more experimental therapeutic approaches.     

Resistance to spontaneous apoptosis in acute myeloid leukaemia blasts is associated with p-glycoprotein expression and function,but not with the presence of FLT3 internal tandem duplications

The ability of acute myeloid leukaemia (AML) blasts to survive in culture has been associated with poor patient response to chemotherapy. Other biological factors predicting an adverse outcome include p-glycoprotein (pgp) expression, which is associated with a reduced remission rate, and the presence of fms-like tyrosine kinase 3 gene (FLT3) internal tandem duplications (ITDs), predictive of a high rate of leukaemic relapse. Our previous work has indicated a drug efflux-independent role for pgp in apoptosis resistance. We measured spontaneous in vitro apoptosis in 58 primary AML samples to establish its relationship with functional and phenotypic pgp and with FLT3 ITDs. Cells were incubated for 48 h in a suspension culture, and the remaining viable cells were counted by flow cytometry. Median survival was 38% of baseline values. Resistance to spontaneous apoptosis was strongly associated with pgp (MRK-16 antibody) expression (P = 0.001) and with pgp functional activity (P < 0.001). FLT3 ITDs, found in 20 cases, were inversely associated with functional pgp activity: thus, the median pgp modulation ratio was 2.0 in FLT3 wild-type cases and 1.38 in ITD cases (P = 0.018). Also, the presence of FLT3 ITDs was not associated with in vitro apoptosis resistance. In conclusion, we have found that the presence of FLT3 ITDs is not related to AML blast survival in vitro, and is inversely associated with pgp activity, whereas pgp expression and activity are associated with resistance to spontaneous apoptosis. These results may help to explain the differing adverse effects of pgp (on remission induction) and FLT3 ITDs (on relapse) in AML.     

The genotype nucleophosmin mutated and FLT3‐ITD negative is characterized by high bax/bcl‐2 ratio and favourable outcome in acute myeloid leukaemia

Nucleophosmin gene (NPM1) mutations characterize acute myeloid leukaemia (AML) with normal karyotype and frequently co‐exist with FLT3 internal tandem duplications (ITD). We evaluated bcl‐2, bax, NPM1 and FLT3‐ITD in 222 AML patients. Bax/bcl‐2 ratio >0·35 and NPM1 without FLT3‐ITD were significantly associated (P = 0·0001). NPM1‐mutated (mt)/FLT3‐ITD negative patients showed a higher complete remission (CR) rate (90%, P = 0·0002) and a longer overall survival (OS, P = 0·00007). NPM1‐mt/FLT3‐ITD negative plus bax/bcl‐2 > 0·35 subset showed a very high CR rate (96%), very long OS (P = 0·00005) and disease‐free survival (P = 0·004). The favourable prognosis of NPM1‐mt/FLT3‐ITD negative patients might be explained by a higher bax/bcl‐2 ratio.     

血液肿瘤患者FLT3/ITD基因突变的检测及临床意义

目的检测血液肿瘤患者FLT3/ITD基因突变,探讨其突变的临床意义。方法2001—2005年对南方医科大学南方医院血液科332例血液肿瘤患者,采用聚合酶链反应(PCR)方法检测FLT3/ITD基因突变。结果FLT3/ITD基因突变阳性率分别为急性髓性白血病(AML)22.3%(23/103)、慢性髓性白血病急变期(CML-BC)6.5%(2/31)、骨髓增生异常综合征(MDS)5.6%(2/36)和急性淋巴细胞白血病(ALL)2.6%(2/76)。而慢性髓性白血病慢性期(CML-CP)、慢性淋巴细胞白血病(CLL)、多发性骨髓瘤(MM)和非霍奇金淋巴瘤(NHL)均未发现FLT3/ITD基因突变。FLT3/ITD基因突变阳性AML患者外周血WBC计数及骨髓白血病细胞比例显著高于FLT3/ITD基因突变阴性AML患者(P<0.05),FLT3/ITD基因突变阳性AML患者完全缓解后18个月内累计复发率(63.6%)显著高于FLT3/ITD基因突变阴性AML组(27.7%)(P<0.05)。结论FLT3/ITD基因突变检测对血液肿瘤预后有一定意义;FLT3/ITD基因突变AML患者预后差。     

A FLT3-targeted tyrosine kinase inhibitor is cytotoxic to leukemia cells in vitro and in vivo

Constitutively activating internal tandem duplication (ITD) and point mutations of the receptor tyrosine kinase FLT3 are present in up to 41% of patients with acute myeloid leukemia (AML). These FLT3/ITD mutations are likely to be important because their presence is associated with a poor prognosis. Both types of mutations appear to activate the tyrosine kinase activity of FLT3. We describe here the identification and characterization of the indolocarbazole derivative CEP-701 as a FLT3 inhibitor. This drug potently and selectively inhibits autophosphorylation of wild-type and constitutively activated mutant FLT3 in vitro in FLT3/ITD-transfected cells and in human FLT3-expressing myeloid leukemia-derived cell lines. We demonstrate that CEP-701 induces a cytotoxic effect on cells in a dose-responsive fashion that parallels the inhibition of FLT3. STAT5 and ERK1/2, downstream targets of FLT3 in the signaling pathway, are inhibited in response to FLT3 inhibition. In primary leukemia blasts from AML patients harboring FLT3/ITD mutations, FLT3 is also inhibited, with an associated cytotoxic response. Finally, using a mouse model of FLT3/ITD leukemia, we demonstrate that the drug inhibits FLT3 phosphorylation in vivo and prolongs survival. These findings form the basis for a planned clinical trial of CEP-701 in patients with AML harboring FLT3- activating mutations.     

In situ RHAMM protein expression in acute myeloid leukemia blasts suggests poor overall survival

Treatment options for patients with high-risk acute myeloid leukemia (AML) include high-dose chemotherapy regimens in combination with allogeneic hematopoietic stem cell transplantation, which takes advantage of the donor T-cell-mediated graft-versus-leukemia effect. Together with beneficial responses observed in assays targeted at leukemia-associated antigens (LAA), this encouraged research on cancer vaccines and adoptive cellular therapies in AML. The receptor for hyaluronic acid-mediated motility (RHAMM, CD168) was identified as one of the most promising LAA in AML. Thus far, little is known about in situ expression in leukemic bone marrow blasts or the prognostic role of RHAMM and its interaction partners in AML. We immunohistochemically analyzed the expression and prognostic significance of RHAMM on trephine bone marrow biopsies from 71 AML cases that had been evaluated for cytogenetics and presence of FLT3-internal tandem duplications and NPM1 mutations. Fifty-five patients (77%) were treated with curative intent, while 16 (23%) received the most appropriate supportive care. Twenty of 71 (28%) AML cases were considered RHAMM+. Receiver operating characteristic curves showed significant discriminatory power considering overall survival (OS) in AML patients treated curatively for RHAMM (p = 0.015). Multivariable analysis revealed that expression of RHAMM in >5% of leukemic blasts identifies a subgroup of curatively treated cases with adverse OS independent of failures to achieve complete remission. RHAMM not only represents a promising LAA with specific T-cell responses in AML but, if assessed in situ on blasts, also a probable prognostic factor.     

FLT3 mutations in myeloid sarcoma

Myeloid sarcoma is an extramedullary tumour that typically occurs in the setting of acute myeloid leukaemia (AML), or myeloproliferative disorders. In AML, two types of mutations in Fms-like tyrosine kinase 3 (FLT3) have been described; internal tandem duplications (ITD) and point mutations at aspartic acid residue 835 (D835). We analysed 24 myeloid sarcoma specimens from 20 patients for FLT3 ITD and D835 mutations. FLT3 ITD mutations were identified in three of 20 cases (15%); no D835 mutations were identified. The ITD inserts ranged in size from 33 to 198 base pairs (bp) and represented approximately 20-40% of the FLT3 alleles. Two cases showed discordance in FLT3 ITD mutational status. In one case, the leukaemia specimen was positive for a FLT3 ITD mutation and the myeloid sarcoma specimen was negative. In the second case, the myeloid sarcoma was positive for a FLT3 ITD mutation at diagnosis, but negative in subsequent relapse samples. Our findings suggest that small molecule inhibitors of FLT3 may be useful therapeutic agents for treatment of myeloid sarcomas-containing FLT3 mutations, however, the potential for discordance between the leukaemia and myeloid sarcoma, necessitates that the myeloid sarcoma tumour itself be analysed for FLT3 mutations.     

Immunophenotypic features of acute myeloid leukaemia patients exhibiting high FLT3 expression not associated with mutations

FMS-related tyrosine kinase 3 (FLT3) mutations are found in 30% of cases of acute myeloid leukaemia (AML). In addition, recent studies have lead to the identification of about 10-15% of AML patients displaying high expression of FLT3, not associated with mutations of the receptor (FLT3 Wild-type High, FLT3WTH). These AMLs, as well as those displaying internal tandem duplication (ITD) are associated with an unfavourable prognosis. However, the biological features of these AMLs are poorly characterized. The present study explored the immunophenotypic features of FLT3WTH AMLs in 94 de novo cases of AML. The levels of FLT3 expression, as assessed by flow cytometry and FLT3 mutational status, was used to identify four AML subgroups: FLT3WTH (14/94); FLT3 Wild-type low (FLT3WTL, 48/94); FLT3 internal tandem duplication (FLT3ITD 26/94); FLT3 aspartic acid 835 (FLT3D835, 6/94). FLT3WTH and FLT3ITD were characterized by: high white blast cell counts; predominance of M4 and M5 French-American-British classification subtypes and associated expression of myelo-monocytic markers; high expression of CD123 and TRAIL-Rs; high expression of receptors for angiogenic growth factors. Addition of FLT3 Ligand to human CD34(+) or monocytic cells stimulated CD123 and TRAIL-R expression. These findings are of potential value for the development of new therapeutic strategies.     

Dasatinib response in acute myeloid leukemia is correlated with FLT3/ITD,PTPN11 mutations and a unique gene expression signature

Novel targeted therapies improve the survival of specific subgroups (defined by genetic variants) of patients with acute myeloid leukemia (AML), validating the paradigm of molecularly targeted therapy. However, identifying correlations between AML molecular attributes and effective therapies is challenging. Recent advances in highthroughput, in vitro drug sensitivity screening applied to primary AML blasts were used to uncover such correlations; however, these methods cannot predict the response of leukemic stem cells. Our study aimed to predict in vitro response to targeted therapies, based on molecular markers, with subsequent validation in leukemic stem cells. We performed ex vivo screening of sensitivity to 46 drugs on 29 primary AML samples at diagnosis or relapse. Using unsupervised hierarchical clustering analysis we identified a group with sensitivity to several tyrosine kinase inhibitors, including the multi-tyrosine kinase inhibitor, dasatinib, and searched for correlations between the response to dasatinib, exome sequencing and gene expression in our dataset and in the Beat AML dataset. Unsupervised hierarchical clustering analysis of gene expression resulted in clustering of dasatinib responders and non-responders. In vitro response to dasatinib could be predicted based on gene expression (area under the curve=0.78). Furthermore, mutations in FLT3/ITD and PTPN11 were enriched in the dasatinib-sensitive samples as opposed to mutations in TP53 which were enriched in resistant samples. Based on these results, we selected FLT3/ITD AML samples and injected them into NSG-SGM3 mice. Our results demonstrate that in a subgroup of FLT3/ITD AML (4 out of 9) dasatinib significantly inhibited leukemic stem cell engraftment. In summary we show that dasatinib has an anti-leukemic effect both on bulk blasts and, more importantly, on leukemic stem cells from a subset of AML patients that can be identified based on mutational and expression profiles. Our data provide a rational basis for clinical trials of dasatinib in a molecularly selected subset of AML patients.     

Mild chronic graft‐versus‐host disease may alleviate poor prognosis associated with FLT3 internal tandem duplication for adult acute myeloid leukemia following allogeneic stem cell transplantation with myeloablative conditioning in first complete remission: a retrospective study

Internal tandem duplication (ITD) of the FLT3 gene (Fms‐like tyrosine kinase 3) is the most commonly found mutation in acute myeloid leukemia (AML). The significance of FLT3‐ITD at diagnosis was retrospectively estimated for allo‐HSCT (allogeneic hematopoietic stem cell transplantation) outcomes in 140 patients, median age of 38, undergoing allo‐HSCT after myeloablative conditioning in first complete remission of AML. FLT3‐ITD was detected at AML diagnosis in 42/140 (30%) of included into this study patients. At 3 years, relapse incidence (RI) following allo‐HSCT in AML patients with intermediate or normal karyotype was significantly higher in those FLT3‐ITD positive than FLT3‐ITD negative [52.9 vs. 20.4%, P = 0.002]. Additionally, patients with mild chronic graft‐versus‐host disease (cGvHD) had significantly lower RI compared to patients with moderate or severe grade cGvHD or those not experiencing cGvHD, respectively, 4.8 vs. 36.0 vs. 27.8%, P = 0.032. FLT3‐ITD was harboring a poor prognosis in AML with intermediate or normal karyotype and significantly increased risk of relapse following allo‐HSCT. It appears that allo‐HSCT does not cure patients with FLT3‐ITD, unless they develop symptoms of mild cGvHD and graft versus leukemia, which may decrease RI.     

Importance of early detection and follow-up of FLT3 mutations in patients with acute myeloid leukemia

Mutations in the fms-like tyrosine kinase 3 (FLT3) gene, such as internal tandem duplication (FLT3/ITD) in the juxtamembrane domain and point mutations in the tyrosine kinase domain, are the most common abnormalities in acute myeloid leukemia (AML). FLT3/ITD and FLT3/D835 mutations were analyzed in 113 Serbian adult AML patients using polymerase chain reaction. Twenty patients were found to be FLT3/ITD positive (17.7%). The mutations occurred most frequently in M5 and M0 subtypes of AML. They were mainly associated with the normal karyotype. All patients harboring FLT3/ITD had a higher number of white blood cells than patients without it (p = 0.027). FLT3/ITD mutations were associated with lower complete remission (CR) rate (χ ²= 5.706; p = 0.017) and shorter overall survival (OS; Log rank = 8.76; p = 0.0031). As for disease-free survival, the difference between FLT3/ITD-positive and FLT3/ITD-negative patients was not statistically significant (Log rank = 0.78; p = 0.3764). In multivariate analysis, the presence of FLT3/ITD mutations was the most significant prognostic factor for both OS and CR rate (p = 0.0287; relative risk = 1.73; 95% CI = 1.06–2.82). However, in the group of patients with the intermediate-risk karyotype, the mere presence of FLT3/ITD was not associated with inferior clinical outcome. FLT3/D835 point mutation was found in four patients (3.5%) only. Follow-up of the FLT3/ITD-positive patients revealed stability of this mutation during the course of the disease. However, changes in the pattern of FLT3/D835 mutations in initial and relapsed AML were observed. Our results indicate an association of FLT3/ITD with the adverse outcome in AML patients treated with standard induction chemotherapy. Because FLT3/ITD mutation is a target for specific therapeutic inhibition, its early detection could be helpful in clinical practice. Ms. Colovic and Ms. Tosic contributed equally to this work.     

Clinical impact of internal tandem duplications and activating point mutations in FLT3 in acute myeloid leukemia in elderly patients

BACKGROUND: The FLT3 gene is frequently mutated in acute myeloid leukemia (AML), either by an internal tandem duplication (ITD) of the juxtamembrane domain or by activating point mutations in the second tyrosine kinase domain (ATKD). Only a few investigations have focused on the prognostic significance of FLT3 alterations in AML among the elderly, yielding conflicting results. In the present study, the frequency and clinical relevance of FLT3 abnormalities were ascertained in a cohort of elderly AML patients. PATIENTS AND METHODS: A total of 109 AMLs, occurring in patients above the age of 60 yr (median 71.5), were investigated. DNA was extracted from fresh bone marrow cells or from cells in fixative and investigated for the presence of ITD of exons 14 and 15 and the ATKD D835 in exon 20. RESULTS: ITDs and ATKDs were identified in 20 (18%) and 11 (10%) of the cases, respectively. Three cases displayed both an ITD and an ATKD. FLT3 abnormalities were associated with leukocytosis (ITD P < 0.01; ATKD P = 0.069), and the monocytic FAB subtypes M4 and M5 [ITD (P < 0.05), ATKD (P = 0.05)], and ITD and ATKD were significantly (P < 0.05) more common in cases with a normal karyotype. There was no correlation between the presence of FLT3 abnormalities and complete remission rates or overall survival. CONCLUSION: A correlation was observed between FLT3 abnormalities and leukocytosis, a normal karyotype, and the M4/M5 subtypes of leukemia. However, no clear-cut prognostic impact of FLT3 abnormalities was identified in elderly AML patients.     

Acute myeloid leukaemia (AML) with t(6;9)(p23;q34) is associated with poor outcome in childhood AML regardless of FLT3‐ITD status: a report from the Children's Oncology Group

Acute myeloid leukaemia (AML) with t(6;9)(p23;q34) is a rare subtype associated with FLT3‐internal tandem duplication (ITD) and poor outcomes. The clinical outcomes of paediatric patients with t(6;9) with and without FLT3‐ITD treated on six consecutive cooperative trails were evaluated. In contrast to patients without t(6;9), those with t(6;9) had a significantly lower complete remission rate, higher relapse rate (RR), and poor overall survival (OS). Within t(6;9) patients, those with and without FLT3‐ITD had an OS of 40% and 27% respectively (P > 0·9), demonstrating that t(6;9) is a high‐risk cytogenetic feature in paediatric AML and its clinical impact is independent of the presence of FLT3‐ITD.