Validation of the 2022 European LeukemiaNet risk stratification for acute myeloid leukemia.

This study aimed to validate the 2022 European LeukemiaNet (ELN) risk stratification for acute myeloid leukemia (AML). A total of 624 newly diagnosed AML patients from 1998 to 2014 were included in the analysis. Genetic profiling was conducted using targeted deep sequencing of 45 genes based on recurrent driver mutations. In total, 134 (21.5%) patients had their risk classification reassessed according to the 2022 ELN risk stratification. Among those initially classified as having a favorable risk in 2017 (n = 218), 31 and 3 patients were reclassified as having intermediate risk or adverse risk, respectively. Among the three subgroups, the 2022 ELN favorable-risk group showed significantly longer survival outcomes than the other groups. Within the 2017 ELN intermediate-risk group (n = 298), 21 and 46 patients were reclassified as having favorable risk or adverse risk, respectively, and each group showed significant stratifications in survival outcomes. Some patients initially classified as having adverse risk in 2017 were reclassified into the intermediate-risk group (33 of 108 patients), but no prognostic improvements were observed in this group. A multivariable analysis identified the 2022 ELN risk stratification, age, and receiving allogeneic hematopoietic cell transplantation as significant prognostic factors for survival. The 2022 ELN risk stratification enables more precise decisions for proceeding with allogeneic hematopoietic cell transplantation for AML patients.

Gilteritinib as Post-Transplant Maintenance for Acute Myeloid Leukemia With Internal Tandem Duplication Mutation of FLT3.

PURPOSE: Allogeneic hematopoietic cell transplantation (HCT) improves outcomes for patients with acute myeloid leukemia (AML) harboring an internal tandem duplication mutation of FLT3 (FLT3-ITD) AML. These patients are routinely treated with a FLT3 inhibitor after HCT, but there is limited evidence to support this. Accordingly, we conducted a randomized trial of post-HCT maintenance with the FLT3 inhibitor gilteritinib (ClinicalTrials.gov identifier: NCT02997202) to determine if all such patients benefit or if detection of measurable residual disease (MRD) could identify those who might benefit. METHODS: Adults with FLT3-ITD AML in first remission underwent HCT and were randomly assigned to placebo or 120 mg once daily gilteritinib for 24 months after HCT. The primary end point was relapse-free survival (RFS). Secondary end points included overall survival (OS) and the effect of MRD pre- and post-HCT on RFS and OS. RESULTS: Three hundred fifty-six participants were randomly assigned post-HCT to receive gilteritinib or placebo. Although RFS was higher in the gilteritinib arm, the difference was not statistically significant (hazard ratio [HR], 0.679 [95% CI, 0.459 to 1.005]; two-sided P = .0518). However, 50.5% of participants had MRD detectable pre- or post-HCT, and, in a prespecified subgroup analysis, gilteritinib was beneficial in this population (HR, 0.515 [95% CI, 0.316 to 0.838]; P = .0065). Those without detectable MRD showed no benefit (HR, 1.213 [95% CI, 0.616 to 2.387]; P = .575). CONCLUSION: Although the overall improvement in RFS was not statistically significant, RFS was higher for participants with detectable FLT3-ITD MRD pre- or post-HCT who received gilteritinib treatment. To our knowledge, these data are among the first to support the effectiveness of MRD-based post-HCT therapy.

Reduced toxicity (FluBu3) versus myeloablative (BuCy) conditioning in acute myeloid leukemia patients who received first allogeneic hematopoietic stem cell transplantation in measurable residual disease-negative CR1.

In the present study, reduced toxicity (FluBu3) and myeloablative (BuCy) conditioning were compared in patients with AML who received first allogeneic HSCT in MRD-negative CR1. The study included 124 adult patients who underwent HSCT from an HLA-matched (8/8) sibling, unrelated, or 1-locus mismatched (7/8) unrelated donor (MMUD). The median age was 45 years and intermediate cytogenetics comprised majority (71.8%). The 2-year OS, RFS, CIR and NRM for BuCy (n = 78, 62.9%) and FluBu3 (n = 46, 37.1%) groups were 78.3% and 84.5% (p = 0.358), 78.0% and 76.3% (p = 0.806), 7.7% and 21.5% (p = 0.074) and 14.3% and 2.2% (p = 0.032), respectively. At the time of data cut-off, relapse and NRM were the main causes of HSCT failure in each of the FluBu3 and BuCy arms. Among patients, 75% of relapsed FluBu3 patients had high-risk features of either poor cytogenetics or FLT3-ITD mutation compared with 16.7% of BuCy patients. The majority of NRM in the BuCy group was due to GVHD (73%), half of whom received MMUD transplantation. To conclude, the FluBu3 reduced toxicity conditioning showed comparable post-transplant OS and RFS to BuCy and was associated with significantly reduced NRM that was offset by a trend towards higher risk of relapse even in MRD-negative CR1 population.

Guadecitabine vs TC in relapsed/refractory AML after intensive chemotherapy: randomized phase 3 ASTRAL-2 trial.

Guadecitabine is a novel hypomethylating agent (HMA) resistant to deamination by cytidine deaminase. Patients with relapsed/refractory acute myeloid leukemia (AML) were randomized to guadecitabine or a preselected treatment choice (TC) of high-intensity chemotherapy; low-intensity treatment with HMAs, or low-dose cytarabine; or best supportive care (BSC). The primary endpoint was overall survival (OS). 302 patients were randomized to guadecitabine (n=148) or TC (n=154). Preselected TCs were low intensity treatment (n=233 [77%; mainly HMAs]), high intensity chemotherapy (n=63 [21%]), and BSC (n=6 [2%]). The median OS were 6.4 and 5.4 months for guadecitabine and TC, respectively (hazard ratio 0.88 [95% confidence interval 0.67, 1.14]; log-rank P=0.33). Survival benefit for guadecitabine was suggested in several prospective subgroups, including age <65 years, Eastern Cooperative Oncology Group performance status 0-1, refractory AML, and lower peripheral blood blasts ≤30%. Complete response (CR) + CR with partial hematologic recovery rates were 17% for guadecitabine vs 8% for TC (P<0.01); CR+CR with incomplete count recovery rates were 27% for guadecitabine vs 14% for TC (P<0.01). Safety was comparable for the two arms, but guadecitabine had a higher rate of grade ≥3 neutropenia (32% vs 17%; P<0.01). This study did not demonstrate an OS benefit for guadecitabine. Clinical response rates were higher for guadecitabine, with comparable safety to TC. There was an OS benefit for guadecitabine in several prespecified subgroups. This study was registered as ClinicalTrials.gov as NCT02920008.

Development of cost-effective and fast KIR genotyping by multiplex PCR-SSP.

Killer-cell immunoglobulin-like receptors (KIR) control natural killer (NK) cell functions by recognizing HLA molecules and modulating the activity of NK cells. The KIR gene cluster contains polymorphic and highly homologous genes. Diversity of the KIR region is achieved through differences in gene content, allelic polymorphism, and gene copy number, which result in unrelated individuals having different KIR genotypes and individualized immune responses that are relevant to multiple aspects of human health and disease. Therefore, KIR genotyping is increasingly used in epidemiological studies. Here, we developed multiplex polymerase chain reaction with sequence-specific primers (PCR-SSP) to compensate for the shortcomings of the conventional PCR-SSP method, which is most commonly used for KIR analysis. Multiplex PCR-SSP method involves six multiplex reactions that detect 16 KIR genes and distinguish variant types of some KIR genes by adding two reactions. The assay was evaluated in a blind survey using a panel of 40 reference DNA standards from the UCLA KIR Exchange Program. The results are 100% concordant with the genotype determined using Luminex-based reverse sequence-specific oligonucleotide typing systems. Additionally, we investigated the currently known 16 KIR genes and their common variants in 120 unrelated Korean individuals. The results were consistent with the KIR genotype previously reported by Hwang et al. This multiplex PCR-SSP is an efficient method for analyzing KIR genotypes in both small- and large-scale studies with minimal labor, reagents, and DNA. Furthermore, by providing a better definition of KIR polymorphisms it can contribute to developments in immunogenetics.

Single-cell RNA Sequencing Reveals Novel Cellular Factors for Response to Immunosuppressive Therapy in Aplastic Anemia.

Aplastic anemia (AA) is a lethal hematological disorder; however, its pathogenesis is not fully understood. Although immunosuppressive therapy (IST) is a major treatment option for AA, one-third of patients do not respond to IST and its resistance mechanism remains elusive. To understand AA pathogenesis and IST resistance, we performed single-cell RNA sequencing (scRNA-seq) of bone marrow (BM) from healthy controls and patients with AA at diagnosis. We found that CD34+ early-stage erythroid precursor cells and PROM1+ hematopoietic stem cells were significantly depleted in AA, which suggests that the depletion of CD34+ early-stage erythroid precursor cells and PROM1+ hematopoietic stem cells might be one of the major mechanisms for AA pathogenesis related with BM-cell hypoplasia. More importantly, we observed the significant enrichment of CD8+ T cells and T cell-activating intercellular interactions in IST responders, indicating the association between the expansion and activation of T cells and the positive response of IST in AA. Taken together, our findings represent a valuable resource offering novel insights into the cellular heterogeneity in the BM of AA and reveal potential biomarkers for IST, building the foundation for future precision therapies in AA.

Dynamic changes in physical function during intensive chemotherapy affect transplant outcomes in older adults with AML.

Introduction: Intensive chemotherapy (IC) can affect all geriatric assessment (GA) domains in older adults with acute myeloid leukemia (AML), but data on the effects of these changes on transplant outcomes are lacking. Methods: Therefore, we prospectively assessed the prognostic role of GA domains at diagnosis and allogeneic hematopoietic stem cell transplantation (allo-HSCT) in 51 patients with AML aged ≥60 years who achieved complete remission after IC. We performed both baseline and pre-allo-HSCT GA; moreover, physical function, including a short physical performance battery (SPPB), cognitive function, psychological function, nutritional status, and social support were examined. Results: All GA domains showed dynamic changes between the two time points. The directions of change were statistically significant for social support, self-reported physical and psychological functions, and distress, but not for nutritional status, cognitive function, or physical function. Among all GA domains at each time point, only poor physical function and its submaneuvers at diagnosis but not at allo-HSCT were significantly associated with inferior survival. In particular, since the direction of change varied between patients, we found that patients whose physical function improved before allo-HSCT were more likely to survive longer than those with persistently impaired SPPB (55.6% vs. 28.6%, p=0.268). Finally, persistent impairment in SPPB (28.6% vs. 65.9%, p=0.006), tandem stand (0% vs. 63.3%, p=0.012), sit-and-stand (41.2% vs. 70.6%, p=0.009), and gait speed (38.5% vs. 68.4%, p=0.027) further strongly predicted inferior survival. Discussion: This study showed that IC courses can induce dynamic changes in different directions in the GA domains of each patient and that changes in objectively measured physical function can predict transplant outcomes.

The salvage role of allogeneic hematopoietic stem-cell transplantation in relapsed/refractory diffuse large B cell lymphoma.

To clarify the role of allogeneic hematopoietic stem-cell transplantation (allo-HSCT) in the chimeric antigen receptor T-cell therapy era, we analyzed the clinical characteristics and outcomes of 52 patients treated with allo-HSCT with relapsed/refractory diffuse large B cell lymphoma. Most enrolled patients had previously undergone intensive treatments, the median number of chemotherapy lines was 4, and the median time from diagnosis to allo-HSCT was 27.1 months. Patients were divided into remission-achieved (n = 30) and active-disease (n = 22) groups before allo-HSCT. Over a median follow-up period of 38.3 months, overall survival (OS) and event-free survival (EFS) rates were 38.4% and 30.6%, respectively. The cumulative incidence of relapse (CIR) and the non-relapsed mortality (NRM) were 36.7% and 32.7%, respectively. OS, EFS, and graft-versus-host disease-free, relapse-free survival (GRFS) outcomes were significantly superior in the remission-achieved group with lower CIR. In a multivariate analysis, a shorter interval from diagnosis to allo-HSCT reflected relatively rapid disease progression and showed significantly poor OS and EFS with higher CIR. Patients with active disease had significantly lower EFS, GRFS, and higher CIR. Previous autologous stem-cell transplantation was associated with better GRFS. Allo-HSCT is an established modality with a prominent group of cured patients and still has a role in the CAR T-cell era, particularly given its acceptable clinical outcomes in young patients with chemo-susceptible disease.

Neuroplastic and Pro-cognitive Effects of Granulocyte Colony Stimulating Factor in Healthy Adults: A Pilot Study.

OBJECTIVE: Granulocyte colony-stimulating factor (G-CSF) is a growth factor used to regulate the mobilization of bone marrow progenitor cells and has been shown to promote brain repair and reduce inflammation. This study aimed to investigate the pro-cognitive and neuroplastic effects of G-CSF in healthy adults. METHODS: Sixteen healthy adults or donors of hematopoietic stem cell transplantation received G-CSF injections for 5 consecutive days, and their blood samples were collected before, immediately after, and 3 weeks after the G-CSF injections. Twelve subjects underwent neuropsychological testing before and 12 weeks after the G-CSF injections. RESULTS: The study found that G-CSF administration resulted in significant improvements in cognitive function, as measured by the Rey- Osterrieth Complex Figure test for immediate recall, delayed recall, and recognition score at 12 weeks after the injections. The blood levels of brain-derived neurotrophic factor, interleukin-4, and interleukin-8 were significantly increased immediately after the injections and returned to baseline levels after 3 weeks. There was no significant change in the plasma level of Multimer Detection System-oligomerized amyloid beta. CONCLUSION: Our results might suggest that G-CSF has neuroplastic and pro-cognitive effects in healthy adults. However, further study containing a larger sample size is needed to confirm our findings.

Effectiveness of leukapheresis on early survival in acute myeloid leukemia: An observational propensity score matching cohort study.

BACKGROUND: The association between leukapheresis (LK) as a treatment option for hyperleukocytosis (HL) in patients with acute myeloid leukemia (AML) remains controversial. METHODS: Data were extracted from the electronic medical record for 2801 patients with AML between April 2009 and December 2019. LK was performed when the leukocyte count was ≥100 × 109 /L at the time initial bone marrow examination. RESULTS: A comparison between the patients with HL in the non-LK (n = 1579) and LK (n = 208) groups revealed survival probabilities (%) of 93.2% and 90.4% (P = .130) for day 30 (D30), 85.4% and 84.2% (P = .196) for D60, and 83.6% and 80.8% (P = .258) for D90, respectively. After propensity score matching, a comparison between the patients with HL in the non-LK (n = 192) and LK (n = 192) groups revealed survival probabilities (%) of 83.9% and 91.2% (P = .030) for D30, 75.0% and 84.9% (P = .015) for day 60 (D60), and 62.4% and 81.3% (P = .034) for day 90 (D90), respectively. After D150, the observed effect of LK appeared to be mitigated without a survival benefit. DISCUSSION: LK was associated with improved early survival outcomes at D30, D60, and D90 among patients with AML exhibiting HL. Thus, it may be considered a treatment option for reducing cell mass in such patients.

Prognostic value of European Leukemia Net 2022 criteria and genomic clusters using machine learning in older adults with acute myeloid leukemia.

This study aimed to validate the new European Leukemia Net (ELN) 2022 criteria for genetic risk stratification in older adults with acute myeloid leukemia (AML) and to determine the most likely set of clusters of similar cytogenetic and mutation properties correlated with survival outcomes in three treatment groups: intensive chemotherapy (IC), hypomethylating agents (HMAs) alone, and HMAs plus venetoclax (HMA/VEN). The study included 279 patients (≥60 years) who received IC (n=131), HMA (n=76), and HMA/VEN (n=72) between July 2017 and October 2021. No significant differences were observed in survival among the groups according to ELN 2022 risk stratification. Unsupervised hierarchical clustering analysis identified nine genomic clusters (C1-9) with varying survival outcomes depending on treatment type. For example, C4 (predominant for core binding factor-AML) displayed a favorable prognosis in the IC group, but not in the HMA or HMA/VEN group. The HMA/VEN group had better outcomes than the HMA group in many clusters (C1, 2, 3, and 5); however, the addition of VEN to HMA or IC did not improve the survival outcomes compared with those of HMA alone in C7 and C9 [predominant for -5, del(5q), -7, -17/abn(17p), complex karyotypes, and mutated TP53 ]. The study highlights the limitations of ELN genetic risk stratification in older adults with AML. It emphasizes the need for a more comprehensive approach that considers co-occurring somatic mutations to guide treatment selection in older adults with AML potentially.

Cytomegalovirus Infection after Allogeneic Hematopoietic Cell Transplantation under 100-Day Letermovir Prophylaxis: A Real-World 1-Year Follow-Up Study.

The prevention and management of cytomegalovirus (CMV) reactivation is important to improve the outcomes of allogeneic hematopoietic cell transplantation (allo-HCT) recipients. The aim of this study was to analyze real-world data regarding the incidence and characteristics of CMV infections until 1 year after allo-HCT under 100-day letermovir prophylaxis. A single-center retrospective study was conducted between November 2020 and October 2021. During the study period, 358 patients underwent allo-HCT, 306 of whom received letermovir prophylaxis. Cumulative incidence of clinically significant CMV infection (CS-CMVi) was 11.4%, 31.7%, and 36.9% at 14 weeks, 24 weeks, and 1 year post-HCT, respectively. Through multivariate analysis, the risk of CS-CMVi increased with graft-versus-host disease (GVHD) ≥ grade 2 (adjusted odds ratio 3.640 [2.036-6.510]; p < 0.001). One-year non-relapse mortality was significantly higher in letermovir breakthrough CS-CMVi patients than those with subclinical CMV reactivation who continued receiving letermovir (p = 0.002). There were 18 (15.9%) refractory CMV infection cases in this study population. In summary, letermovir prophylaxis is effective at preventing CS-CMVi until day 100, which increased after the cessation of letermovir. GVHD is still a significant risk factor in the era of letermovir prophylaxis. Further research is needed to establish individualized management strategies, especially in patients with significant GVHD or letermovir breakthrough CS-CMVi.

Limited efficacy of Venetoclax Combination Regimens in Acute Myeloid Leukemia with Extramedullary Relapse.

The recently approved Bcl-2 inhibitor venetoclax (VEN) has achieved promising outcomes in new and relapsed/refractory (R/R) acute myeloid leukemia (AML). Although its use is not well-established in R/R AML with extramedullary disease (EMD), some reports have shown promising outcomes. We retrospectively analyzed 11 patients of R/R AML with EMD [with (n=4) or without (n=7) concurrent marrow involvement], who were treated with VEN plus decitabine (n=9) or low-dose cytarabine (n=2) between May 2020 and October 2020 in Seoul St. Mary's Hospital. The median number of prior treatment lines was 3 (1-6) and most (n=9, 81.8%) had multiple sites of EMD. Nine patients (81.8%) received concurrent therapy for EM involvement sites with radiotherapy (RT) (n=4), surgery (n=1) and both of them (n=4). Among 11 patients, four patients (36.4%) had either marrow or EM responses to VEN-combination; EM response was seen in one patient (9.1%, partial response) who had received concurrent radiotherapy (25Gy, 10fx) during 1st cycle of VEN-combination, and other three patients showed marrow response without EM response. After median follow-up of 27.0 months, the median overall survival was estimated to be 5.4 months. To conclude, VEN-combination regimens have shown only modest efficacy in EM recurrence of AML with little impact on eliciting EM response.

Efficacy of inhaled tiotropium add-on to budesonide/formoterol in patients with bronchiolitis obliterans developing after hematopoietic stem cell transplantation.

BACKGROUND: Bronchiolitis obliterans syndrome (BOS) is the lung manifestation of chronic graft-versus-host disease after hematopoietic stem cell transplantation (HSCT). We assessed whether inhaled tiotropium add-on to the combination regimen including budesonide/formoterol improve pulmonary function and the chronic obstructive pulmonary disease assessment test (CAT) scores in patients with BOS. METHODS: Post-HSCT patients diagnosed as BOS in Seoul St. Mary's Hospital were reviewed retrospectively. Patients defined as BOS and treated with budesonide/formoterol/tiotropium combination therapy after budesonide/formoterol therapy from January 2011 to June 2019 were enrolled. RESULTS: Total of 86 patients were evaluated. After tiotropium add-on, the absolute FEV1 increased significantly from 1.47 ± 0.49 to 1.53 ± 0.57 L (p = 0.023) and the % predicted FEV1 from 45.0 ± 12.8 to 46.8 ± 14.5% (p = 0.031). The % predicted DLCO increased significantly after tiotropium add-on (from 61.6 ± 16.7 to 64.3 ± 16.3%, p = 0.028). Among 56 patients with complete CAT scores, no significant change was present in total CAT scores. In all, 30 of the 72 patients (41.7%) evidenced FEV1 increases > 100 mL, and 20 of 56 patients (35.7%) had CAT score decreases of ≥ 2 points. When the FEV1 and CAT scores were combined, the overall response rate to tiotropium add-on was 56.2% (41/73). The response group evidenced a significantly greater FVC increase, and a significant decrease in the RV/TLC ratio compared to the no-response group. CONCLUSIONS: Inhaled tiotropium add-on to combination budesonide/formoterol significantly improved lung function, but not respiratory symptoms, in patients with post-HSCT BOS.

Machine Learning Predicts 30-Day Outcome among Acute Myeloid Leukemia Patients: A Single-Center, Retrospective, Cohort Study.

Acute myeloid leukemia (AML) is a clinical emergency requiring treatment and results in high 30-day (D30) mortality. In this study, the prediction of D30 survival was studied using a machine learning (ML) method. The total cohort consisted of 1700 survivors and 130 non-survivors at D30. Eight clinical and 42 laboratory variables were collected at the time of diagnosis by pathology. Among them, six variables were selected by a feature selection method: induction chemotherapy (CTx), hemorrhage, infection, C-reactive protein, blood urea nitrogen, and lactate dehydrogenase. Clinical and laboratory data were entered into the training model for D30 survival prediction, followed by testing. Among the tested ML algorithms, the decision tree (DT) algorithm showed higher accuracy, the highest sensitivity, and specificity values (95% CI) of 90.6% (0.918-0.951), 70.4% (0.885-0.924), and 92.1% (0.885-0.924), respectively. DT classified patients into eight specific groups with distinct features. Group 1 with CTx showed a favorable outcome with a survival rate of 97.8% (1469/1502). Group 6, with hemorrhage and the lowest fibrinogen level at diagnosis, showed the worst survival rate of 45.5% (25/55) and 20.5 days. Prediction of D30 survival among AML patients by classification of patients with DT showed distinct features that might support clinical decision-making.

Efficacy of ex vivo purging with CD34+ selection to maximize the effects of autologous stem cell transplantation in peripheral T-cell lymphoma patients.

BACKGROUND AIMS: Peripheral T-cell lymphomas (PTCLs) are rare and aggressive tumors with uncertain optimal treatment. This study investigated the clinical outcomes of high-dose chemotherapy (HDT) followed by autologous stem cell transplantation (ASCT) after CD34+ selective purging in PTCL patients. METHODS: Retrospective analysis included 67 PTCL patients who achieved remission and underwent HDT/ASCT. CD34+ selective purging was performed using CliniMACS® (Miltenyi Biotec, Bergisch Gladbach, Germany). Survival outcomes, engraftment, lymphocyte subsets and viral infections were evaluated. RESULTS: CD34+ selective purged autografts were associated with significantly improved overall survival (OS) and disease-free survival (DFS) compared with unpurged autografts (5-year OS, 73.3% versus 37.8%, 5-year DFS, 73.8% versus 33.4%). The cumulative incidence of relapse was also lower in the purged group (31.5% versus 73.3%). Subgroup analysis revealed significant survival benefits in the high-risk group receiving purged autografts. Lymphocyte subset analysis showed increased natural killer (NK) cell counts in the purged group after ASCT. Higher post-ASCT lymphocyte-to-monocyte ratio (LMR) was associated with improved OS and DFS. CONCLUSIONS: CD34+ selective purging in PTCL patients undergoing HDT/ASCT improved survival outcomes and reduced relapse risk. The procedure increased NK cell counts and post-ASCT LMR. CD34+ selective purging may minimize autograft tumor cell contamination and enhance efficacy in T-cell lymphomas.

FLT4 as a marker for predicting prognostic risk of refractory acute myeloid leukemia.

Treating patients with refractory acute myeloid leukemia (AML) remains challenging. Currently there is no effective treatment for refractory AML. Increasing evidence has demonstrated that refractory/relapsed AML is associated with leukemic blasts which can confer resistance to anticancer drugs. We have previously reported that high expression of Fms-related tyrosine kinase 4 (FLT4) is associated with increased cancer activity in AML. However, the functional role of FLT4 in leukemic blasts remains unknown. Here, we explored the significance of FLT4 expression in leukemic blasts of refractory patients and mechanisms involved in the survival of AML blasts. Inhibition or absence of FLT4 in AML blasts suppressed homing to bone marrow of immunocompromised mice and blocked engraftment of AML blasts. Moreover, FLT4 inhibition by MAZ51, an antagonist, effectively reduced the number of leukemic cell-derived colony-forming units and increased apoptosis of blasts derived from refractory patients when it was co-treated with cytosine arabinoside under vascular endothelial growth factor C, its ligand. AML patients who expressed high cytosolic FLT4 were linked to an AML-refractory status by internalization mechanism. In conclusion, FLT4 has a biological function in leukemogenesis and refractoriness. This novel insight will be useful for targeted therapy and prognostic stratification of AML.