FLT3-ITD Positive Acute Myeloid Leukemia Research Articles

Outcomes of Children and Adolescents with Acute Myeloid Leukemia and FLT3-ITD Treated with Low- or Standard-Dose Chemotherapy Plus Sorafenib on the Cals III-AML18 Trial (ChiCTR1800015883)

Background: FLT3 -ITD mutations are linked to a poor prognosis in pediatric acute myeloid leukemia (AML). Combining targeted therapies, such as sorafenib, with chemotherapy may enhance outcomes for these patients. Low-dose chemotherapy (LDC) has demonstrated safety and effectiveness in some cases. We aimed to investigate whether combining LDC or standard-dose chemotherapy (SDC) with sorafenib can improve the prognosis of patients with FLT3-ITD-positive AML. Methods: From June 2018 to June 2022, this multicenter clinical trial enrolled 497 children with newly diagnosed acute myeloid leukemia (AML), who were randomly assigned to receive either low-dose chemotherapy (LDC) or standard-dose chemotherapy (SDC). Among these, 60 patients were positive for FLT3-ITD mutations. Sorafenib was administered based on recommendations of physicians at a dosage of 200 mg/m²/day. We compared clinical characteristics and outcomes between patients receiving sorafenib (n=40) and those not receiving sorafenib (n=20), focusing on rates of complete morphologic remission with or without platelet recovery (CR/CRi), overall survival (OS), and event-free survival (EFS). Results: There were no significant differences in clinical and biological characteristics between the sorafenib and non-sorafenib groups. Of the 60 patients with AML harboring FLT3-ITD mutations, 33 received sorafenib starting from Induction I. After Induction I, complete morphologic remission with or without platelet recovery (CR/CRi) was achieved in 48.5% of patients in the sorafenib group and 59.3% in the non-sorafenib group (P = .405). However, the CR rate was significantly higher in the for the patients in the LDC group compared to the ones treated with SDC (73.3% vs. 33.3%, P = .002), regardless of sorafenib administration. The median time to neutrophil count recovery was 24 days for patients receiving sorafenib compared to 19.5 days for those not receiving sorafenib after Induction I (P = .0482). The median time to platelet count recovery was 16 days for the sorafenib group and 17.5 days for the non-sorafenib group (P = .0938). Sorafenib did not improve OS or EFS). Patients in the sorafenib group had a 3-year OS of 84.5% ± 5.8% compared to 69.9% ± 11.6% in the non-sorafenib group (P = .336) and a 3-year EFS of 53.1% ± 8.2% versus 54.0% ± 11.4% (P = .862). Eighty-five percent of patients underwent allogeneic hematopoietic stem cell transplantation (HSCT) during their first complete remission (CR1), which significantly improved OS and EFS in patients with FLT3-ITD AML (P = .000). The combination of sorafenib with LDC significantly improved 3-year EFS compared to SDC (66.9% vs. 40.0%, P = .030). In multivariate analysis, sorafenib was not associated with improved outcomes in FLT3-ITD-positive pediatric AML. Conclusion: Patients with FLT3-ITD-positive AML treated with LDC with and without sorafenib achieved favorable remission. Allogeneic hematopoietic stem cell transplantation (HSCT) during first complete remission (CR1) significantly improved overall survival (OS) and event-free survival (EFS). However, sorafenib did not show a significant improvement in prognosis for pediatric patients with FLT3-ITD positive AML in this study, indicating that further randomized controlled trials may be needed to confirm its efficacy.

Clonal Dynamics and Relapse Risk Revealed by High-Sensitivity FLT3-Internal Tandem Duplication Detection in Acute Myeloid Leukemia

The ability to detect low-level disease is key to our understanding of clonal heterogeneity in acute myeloid leukemia (AML) and residual disease that elude conventional assays and seed relapse. We developed a high-sensitivity next-generation sequencing (HS-NGS) clinical assay, able to reliably detect low levels (1 × 10−5) of FLT3-ITD, a frequent, therapeutically targetable and prognostically relevant mutation in AML. By applying this assay to 289 longitudinal samples from 62 patients at initial diagnosis and/or clinical follow-up (mean follow-up of 22 months), we reveal the frequent occurrence of FLT3-ITD subclones at diagnosis and demonstrate a significantly decreased relapse risk when FLT3-ITD is cleared after induction or thereafter. We perform pairwise sequencing of diagnosis and relapse samples from 23 patients to uncover more detailed patterns of FLT3-ITD clonal evolution at relapse than is detectable by less-sensitive assays. Finally, we show that rising ITD level during consecutive biopsies is a harbinger of impending relapse. Our findings corroborate the emerging clinical utility of high-sensitivity FLT3-ITD testing and expands our understanding of clonal dynamics in FLT3-ITD–positive AML.

Patient-Reported Outcomes in Acute Myeloid Leukemia Patients with FLT3-ITD Mutation Receiving Quizartinib Vs. Standard Chemotherapy: Results from the Quantum-First Trial

Introduction: QuANTUM-First (NCT02668653) is a multicenter, global, randomized, double-blind phase 3 clinical trial evaluating the efficacy and safety of the novel oral fms-like tyrosine kinase 3 inhibitor (FLT3i) quizartinib in combination with standard induction and consolidation chemotherapy, and as maintenance monotherapy for up to 36 cycles in patients 18-75 years old with FLT3-ITD positive acute myeloid leukemia (AML). Quizartinib demonstrated a clinically meaningful and statistically significant improvement in overall survival (OS) versus placebo. An exploratory endpoint of the trial was to assess the impact of quizartinib on patient-reported outcomes (PROs). This is the first report of the longitudinal results of the PRO measures (PROMs) collected in the QuANTUM-First trial. Methods: Patients were randomized to receive chemotherapy + quizartinib or chemotherapy + placebo. PROMs were the European Organization for Research and Treatment of Cancer QLQ-C30 (EORTC QLQ-C30) and EuroQol EQ-5D-5L. Baseline measurement was collected on day 8 of the first induction cycle, and repeated measurements were collected on day 28 of induction cycles 1-2, day 6 and 28 of consolidation cycles 1-4, and day 1 of continuation cycles 1-34 at 3-cycle intervals. EORTC QLQ-C30 and EQ-5D-5L scores were calculated as per their respective scoring manuals and included mean (95% CI, p-value) score for each domain of the EORTC QLQ-C30 and EQ-5D-5L index score, as well as mean change from baseline score at each timepoint. A minimal clinically important difference (MCID) score ≥10 points for each domain of the EORTC QLQ-C30, was considered clinically meaningful. A mixed-effect model for repeated measures (MMRM) and time until definitive deterioration (TUDD) was used to assess the longitudinal impact of treatment on PROs. TUDD was defined as time from baseline PRO score to first deterioration of the score beyond the MCID as compared to the baseline without further improvement of more than one MCID as compared to the reference score or without any further available score. Results: A total of 509 out of 539 patients from the trial (254 quizartinib; 255 placebo) were included for PRO analysis. Patient compliance to questionnaire completion at the beginning of each phase was high and similar for both arms and most scales (Global Health Status [GHS] completion rate (%) on induction cycle 1, consolidation cycle 1, and continuation cycle 1: 99.2, 95.3, 93.4, respectively) Baseline PRO scores were comparable between arms and were lower than the general population norm EORTC QLQ-C30 and EQ-5D-5L scores for EU and US (Table 1). Per MMRM, there were clinically meaningful improvements from baseline in GHS (Figure 1) and fatigue in both arms. However, there was no significant difference between arms for the change from baseline score although placebo arm had numerically better scores specifically during the maintenance phase (treatment difference for quizartinib - placebo was -2.0 [95% CI: -4.8, 0.7], p=0.1479 for GHS and 3.0 [95% CI: -0.1, 6.1], p=0.0600 for fatigue). Similar trends towards clinically meaningful improvement in score over time for both arms and no significant differences between arms were observed in most of the functional and symptom scales of EORTC QLQ-C30 and EQ-5D-5L index scores. By TUDD analysis, quizartinib arm showed slower deterioration in several scales (specifically GHS, cognitive function, appetite loss, and constipation) but the differences did not reach statistical significance. Conclusions: QuANTUM-First is the first study to explore the impact of quizartinib on PROs. Quizartinib showed improvement in OS without any detrimental impact on quality of life and symptoms when added to standard chemotherapy followed by maintenance monotherapy in patients with newly diagnosed FLT3-ITD AML.

Characteristics of FLT3-ITD and Efficacy of Sorafenib Treatment in Pediatric AML: A Report from AML-BFM Study Group

Introduction Internal tandem duplication (ITD) of FMS-related tyrosine kinase 3 ( FLT3) gene is one of the most common somatic mutations in acute myeloid leukemia (AML). We and others have previously shown that the prognostic significance of FLT3-ITD in children and young adults relies on its characteristics e.g., co-occurrence with WT1 mutationsand NUP98::NSD1 fusions (Niktoreh et al. 2019) or higher allelic ratio (AR) (Tarlock et al. 2018) lead to worse survival outcomes. Also, recent studies suggested that treatment with FLT3 inhibitors, including sorafenib, might be effective in children with FLT3-ITD positive AML. In this report, we summarize the characteristics of FLT3-ITD and outcomes of treatment with sorafenib in patients recruited in AML-BFM consecutive trials. Methods Patients aged 0-21 years old, diagnosed with FLT3-ITD positive de novo AML between Jan. 2004 and Dec. 2021 in Germany and recruited in AML-BFM 2004 and 2012 studies or the 2012 and 2017 registries were retrospectively analyzed. All patients with FLT3-ITD with high- or intermediate risk stratification (HR or IR) were recommended to receive 100 mg/m² sorafenib taken orally twice daily on days 7 to 21 of their induction and consolidation chemotherapy blocks, the final decision however was made by the treating physicians. Patient samples were examined in AML-BFM reference laboratory in Essen, Germany using standard methods. Patients with Down syndrome and AML-M3 were excluded from the current analysis and survival rates were calculated using Kaplan-Meier method and Log-Rank test (significance threshold p<0.05). Results In total, 137 patients (51% male) with median age of 7.78 (1.3-19, IQR=7.27) years at diagnosis were included. Most of the patients were initially stratified in IR group (70%, N=95) followed by HR (26%, N=35). FABs M2, M4 and M1 consisted 74% of all morphologic subtypes with 26%, 25% and 23% of patients, respectively and with a median of 64,000 (500 -700,400, IQR=53,178) x10 9/L, the majority of patients showed a leukocytosis at diagnosis. The median ITD length in the 107 patients with available data was 51 (9-111, IQR=39) bps and 54% of the 91 patients with available data had an AR above 0.5. WT1 mutations and NUP98::NSD1 were detected in 25% (N=34) and 7% (N=10) of patients, respectively and 4% (N=6), harbored both mutations simultaneously. The three-year probability of overall and event-free survival (3y-OS and 3y-EFS, respectively) in all FLT3-ITD patients were 68.8% (±4.3) and 47.6% (±4.5), respectively and although harboring ITDs longer than the median length had a tendency to worsen the 3y-OS (54.4% in ITD>51 bps vs. 73.3% in ITDs<51 bps, p=0.071), it did not influence the 3-y EFS (41.7% vs. 54.2%, p=0.204). Similarly, an AR higher than 0.5 did not affect the survival outcomes (3y-OS: 67.4% vs. 60.4%, p=0.744, 3y-EFS: 46.8% vs. 47%, p=0.618). In total, 36 patients (27%) were treated with sorafenib of which, 23 (63%) received a consecutive HSCT. We observed no survival advantage between sorafenib-treated and sorafenib-naive groups of patients (3y-OS: 73.7% vs. 67.3%, p=0.408, 3y-EFS: 48.3% vs. 47.5%, p=0.929). Patients with longer ITDs did not benefit from sorafenib treatment (3y-OS: 68% vs. 54.3%, p=0.279, 3y-EFS: 48.9% vs. 38.4 %, p=0.649) but higher AR, showed a tendency of improved survival outcomes when sorafenib treatment was implicated (3y-OS: 80.8 % vs. 61.9 %, p=0.159, 3y-EFS: 67.3 % vs. 39.9 %, p=0.131). Although the number of patients were limited, we observed a possible survival advantage in double or triple FLT3-ITD, NUP98::NSD1 and WT1 mutated patients who received Sorafenib: from 28 patients with WT1 mutations only, four patients received sorafenib three of which, are currently alive and in CR but contrarily, from the 24 untreated patients, 14 (58%) are deceased (78% due to relapse). Additionally, from 10 patients with NUP98::NSD1 (w/o WT1 mutations), two who were treated with sorafenib are alive and in CR, whereas from the remaining 8 patients, 6 (75%) experienced relapse and unfortunately did not survive. Conclusions Our preliminary analysis implicates that although addition of sorafenib to the intensive chemotherapy of children and adolescence with FLT3-ITD positive AML, did not improve the survival outcomes in allpatients; specific genetic subgroups might benefit from this treatment. These results must be validated in bigger cohorts of patients with longer follow-ups.

Clonal Dynamics of FLT3-ITD Positive Acute Myeloid Leukemia Patients with Relapsed/Refractory Disease Following Intensive Chemotherapy +/- Midostaurin

Introduction: Despite the wider use of midostaurin (MIDO) in combination with intensive chemotherapy (ICT) as the 1st-line treatment for FLT3-mutated acute myeloid leukemia (AML), complete remission (CR) rates are close to 60%, and relapses occur in over 40% of cases, demonstrating the ability of leukemic cells to resist and evade therapy ( Stone et al., NEJM 2017). Conventional fragment-length analyses of paired diagnosis/relapse samples have shown that FLT3-internal tandem duplications (ITDs) are retained in 80% and 50% of cases following ICT alone and MIDO+ICT respectively ( Schmalbrock et al., Blood 2021). Only limited data are available on the dynamics of FLT3-ITDs or other co-mutations in refractory patients (pts). Here, we conducted a retrospective study involving 115 pts with relapsed/refractory AML harboring FLT3-ITD at diagnosis. Materials and methods: Clonal evolution was examined in paired diagnosis/progression blood or bone marrow samples from 115 pts with FLT3-ITD+ AML treated with MIDO+ICT (n=33) or ICT alone (n=82). Among them, 21 pts had primary refractory disease (MIDO+ICT, n=8; ICT, n=13) and 94 pts relapsed after achieving CR (MIDO+ICT, n=25; ICT, n=69). FLT3-ITDs and co-mutations were screened on genomic DNA by high-throughput sequencing at both timepoints using a custom-designed panel. For accurate annotation and quantification of FLT3-ITDs from sequencing data, we used the recentlypublished FiLT3r algorithm ( Boudry et al., BMC Bioinformatics 2022). For each ITD detected, FiLT3r allelic ratio (AR) was assessed by the ratio between the mutated allele and the wild-type allele. Results: A total of 226 FLT3-ITDs were detected in 115 pts at AML diagnosis, among which 120 (53%) ITDs were found with an AR below 0.05 ( Figure 1). Among pts who achieved CR and experienced relapse (n=94), 48 had multiple FLT3-ITDs at diagnosis and 46 had a single FLT3-ITD at diagnosis. Overall, we observed a simplification of the FLT3-ITD repertoire upon relapse with the persistence of at least one initial clone in 8/12 [67%] and 24/36 [67%] pts with multiple ITDs receiving MIDO+ICT and ICT alone respectively. In relapsed pts who initially had a single FLT3-ITD clone at diagnosis, the addition of MIDO to ICT was associated with a higher rate of FLT3-ITD negativity compared to pts receiving ICT alone (6/13 [46%] vs 5/33 [15%]; P = 0.05) ( Figure 2). Interestingly, among 21 pts with primary refractory AML, we observed that FLT3-ITD mutation status became negative in 5/8 pts (62%) and 2/13 pts (15%) after induction with MIDO+ICT and ICT alone respectively. We then compared the initial characteristics between retained and lost FLT3-ITDs at AML relapse. Lost FLT3-ITDs had significantly lower AR than retained clones in both treatment groups. In order to limit the impact of sample dilution on the allele burden, we defined adjusted variant allele frequencies (VAFs) as the VAFs of FLT3-ITDs normalized to the VAFs of NPM1 mutations, whenever applicable. In so doing, we observed that adjusted VAFs of retained FLT3-ITDs increased at relapse, regardless of the treatment group (adjusted VAF, diagnosis vs relapse: 0.28 vs 0.86 and 0.88 vs 1.6 in the MIDO+ICT group and ICT alone group; P = 2.3e-03 and P = 2.4e-04). Importantly, an adjusted VAF higher than 1 was strongly suggestive of a homozygous state of FLT3-ITD. Such situation was found to be more prevalent at relapse in both treatment groups. Besides the selection of a dominant FLT3-ITD clone, other relapse-related changes including the acquisition of additional mutations will be presented. Conclusion: Our study of the clonal dynamics of AML with FLT3-ITD mutations provides insights into the mechanisms underlying therapy escape. Our data suggest that clonal interference characterized by multiple FLT3-ITD clones is associated with a greater ability to select a FLT3-ITD-positive clone at relapse in pts receiving MIDO+ICT. Although the addition of MIDO to ICT increases the probability of eradicating a single FLT3-ITD clone, FLT3-ITD+ relapses remain common following this combination, often with the selection of homozygous FLT3-ITD clones and/or the emergence of new mutations. Finally, our data in refractory situation emphasize the need to reassess mutational status at each stage of progression before implementing targeted therapy.

The Diversity of FLT3-ITD and the Prognostic Impact in Adult Acute Myeloid Leukemia

Introduction FLT3 internal tandem duplication (ITD) is commonly detected in patients with acute myeloid leukemia (AML) and is known to confer poor prognosis. It has different insertion numbers, mutation burden, insertion sizes, insertion sites and insertion nucleotide sequences, showing complex diversity. However, the guideline of AML did not sufficiently consider the impact of the diversity of FLT3-ITD on treatment responses, and previous studies have not been consistent with the impact of FLT3-ITD diversity on clinical prognosis. In this study we examined the diversity of FLT3-ITD and correlated these parameters with clinical outcome in adult AML. In addition, we revealed the correlation between FLT3-ITD and drug resistance through evolutionary trajectories in primary resistant patients. Methods We included adult patients from 2016-2021 at Henan Cancer Hospital with newly diagnosed adult AML. next generation sequencing (NGS) was performed for all patients at diagnosis. The FLT3-ITD was also detected using conventional methods based on PCR, with a cut-off of 3% for positivity. The comparative analysis of NGS and PCR was done to validate the diversity of FLT3-ITD. Whole genome sequencing (WGS) data for multiple time points were used to analyze clonal evolution. Data were collected by chart review. The chi-squared or Fisher's exact test, Mann-Whitney U test, log-rank and Cox regression analyses were performed. Results Among 406 newly diagnosed adult AML patients, 24.4% (99/406) patients were detected with FLT3-ITD. The FLT3-ITD diversity was validated by NGS and PCR data from 90 FLT3-ITD positive patients. 61.1% (55/90) patients had single FLT3-ITD (sFLT3-ITD) (31M/24F, median age 46 at diagnosis, range 18-64), and 38.9% (35/90) patients carried multiple FLT3-ITDs (mFLT3-ITD) at the same time (23M/12F, median age 47 at diagnosis, range 19-64). A total of 161 FLT3-ITDs were detected, with a median mutation burden of 21.51% (0.71-93.21%) and a median insertion length of 36bp (9-189 bp). 80.20% of FLT3-ITD insertions were in the near-membrane region, and 45.54% of FLT3-ITD insertions contained exogenous nucleic acid sequences. Patients with mFLT3-ITD had a significantly lower CR rate than those with sFLT3-ITD (25.7% vs 60.0%, p=0.001), while the CR rate of patients with sFLT3-ITD was similar to patients without FLT3-ITD (60.0% vs 58.6%, p=0.521). In the setting of NPM1, the CR rate of patients with sFLT3-ITD+NPM1 mut improved significantly when compared to patients with sFLT3-ITD+NPM1 wild (76.9% vs 44.8%, p=0.015). Unfortunately, the state of NPM1 had no impact on the CR rate of patients with mFLT3-ITD. After a median follow-up of 38.9 months, the median overall survival (OS) has not been reached for patients with a single FLT3-ITD, and the median OS for patients with multiple FLT3-ITDs was only 12.4 months (95 CI, 3.8 to 20.9) ( p=0.028). Multivariate analysis confirmed its prognostic significance on CR rate (HR 2.13, 95% CI, 1.38-4.20, p=0.020) but not on OS. The mutation burden, insertion sizes, insertion sites and insertion nucleotide sequences of FLT3-ITD had no significant impact on the CR rate or OS. To explore the clonal architecture of mFLT3-ITD, single cell DNA sequencing on one patient showed that respective FLT3-ITD variants were detected in completely separated cell populations. This suggested that mFLT3-ITD independently occurred in different subclone. WGS data of persistent primary resistant patients at multiple time points were used to analyze the evolutionary trajectory under chemotherapy. We observed complete separation of evolving trajectory of different FLT3-ITD variants over the course of chemo-treatment. Importantly, FLT3-ITD subclone of diagnosis evolved into the dominant clone at resistance. Conclusion In summary, by exploring the impact of the FLT3-ITD diversity on clinical prognosis and the evolutionary trajectory of mFLT3-ITD, the FLT3-ITD number may be a biomarker to refine the risk stratification and further identify adverse populations among FLT3-ITD positive AML patients.

Preliminary study on the regulation of acute myeloid leukemia by FLT3 gene expression

To assess the influence of FLT3 expression on the prognosis of patients with acute myeloid leukemia (AML) by cell experiment and clinical data analysis. Models for FLT3 over-expression and interference-expression in AML cells were constructed. The level of BAK gene expression and its protein product was determined, along with the proliferation and apoptosis of leukemia cells. FLT3 gene expression and FLT3-ITD variant were determined among patients with newly diagnosed AML. Compared with the interference-expression group, the level of BAK gene expression and its protein in FLT3 over-expression AML cells was significantly lower (P < 0.001), which also showed significantly faster proliferation (P < 0.001) and lower rate of apoptosis (P < 0.001). The expression level of FLT3 gene among patients with newly diagnosed AML was also significantly higher compared with the healthy controls (P < 0.001). The FLT3 gene expression of FLT3-ITD positive AML patients was higher than that of FLT3-WT patients (P = 0.002). Survival analysis showed that AML patients with high FLT3 expression in the medium-risk group had a lower complete remission rate and overall survival rate compared with those with a low FLT3 expression (P < 0.001). Over-expression of FLT3 may influence the course of AML by promoting the proliferation of leukemia cells and inhibiting their apoptosis, which in turn may affect the prognosis of patients and serve as a negative prognostic factor for AML.

A key role of the WEE1-CDK1 axis in mediating TKI-therapy resistance in FLT3-ITD positive acute myeloid leukemia patients

The insertion site of the internal tandem duplications (ITDs) in the FLT3 gene affects the sensitivity to tyrosine kinase inhibitors (TKIs) therapy in acute myeloid leukemia (AML). Patients with the ITD in the tyrosine kinase domain lack effective therapeutic options. Here, to identify genotype-driven strategies increasing the TKI therapy efficacy, we developed SignalingProfiler, a strategy supporting the integration of high-sensitive mass spectrometry-based (phospho)proteomics, RNA sequencing datasets with literature-derived signaling networks. The approach generated FLT3-ITD genotype-specific predictive models and revealed a conserved role of the WEE1-CDK1 axis in TKIs resistance. Remarkably, pharmacological inhibition of the WEE1 kinase synergizes and strengthens the pro-apoptotic effect of TKIs therapy in cell lines and patient-derived primary blasts. Finally, we propose a new molecular mechanism of TKIs resistance in AML and suggest the combination of WEE1 inhibitor and TKI as a therapeutic option to improve patients clinical outcome.

Implementing RNAi Therapy Targeting FLT3 in Acute Myeloid Leukemia (AML)

Approximately 25% of newly diagnosed AML patients display an internal tandem duplication (ITD) in the fms-like tyrosine kinase 3 (FLT3) gene. Although both multi-targeted and FLT3 specific tyrosine kinase inhibitors (TKIs) are being utilized for clinical therapy, drug resistance, short remission periods, and high relapse rates are challenges that still need to be tackled (Gebru et. al. J. Hematol. Oncol. 2020). RNA interference (RNAi), mediated by short interfering RNA (siRNA) presents a novel therapeutic platform with the potential of personalization due to its gene sequence-driven mechanism of action. The success of RNAi therapy to a great extent is dictated by the siRNA delivery vehicle employed. This study explored the use of a lipid-substituted low molecular weight polyethyleneimine (lipopolymer) for delivering FLT3 siRNA in FLT3-ITD positive AML models and assess the efficacy of combining this treatment with chemotherapeutic drugs currently used in clinic. Low molecular weight polyethyleneimine (PEI) polymers were modified with aliphatic lipids to yield lipopolymers that self-assembled into nanocomplexes with siRNA. Treatment of MV4-11 cells with lipopolymer/FLT3 siRNA nanocomplexes resulted in ~50% reduction in cell proliferation rates (p<0.001), as determined by taking direct cell counts after the siRNA treatment. Quantitative analysis of relative mRNA transcript levels conducted by qPCR revealed downregulation of FLT3 gene by 30% (p<0.01). Furthermore, a similar decline (~35%, p<0.005) in FLT3 protein levels was observed when measured by flow cytometry through immunostaining with an anti-FLT3 antibody. Evaluation of apoptosis by FITC-Annexin V/propidium iodide (PI) staining showed a 3-fold increase in percentage of cells in the late apoptosis stage after FLT3 specific siRNA treatment (p<0.005). The effect of combining lipopolymer/FLT3 siRNA treatment with daunorubucin manifested as a decline in cell proliferation rates as well as 3-fold increase in apoptosis (p<0.05). To assess the impact of combination treatment on stem cells, colony forming ability was measured by seeding treated cells in methylcellulose and the colonies generated subsequently were counted after two weeks. A notable drop in colony counts was observed in the combination relative to individual treatments. However, similar analyses with the FLT3 targeting TKI midostaurin led to only a modest augmentation of anti-leukemic activity. Subsequently, the more recently approved FLT3 targeting TKI gilteritinib was explored in combination with lipopolymer/FLT3 siRNA nanocomplexes both in vitro and in vivo. For in vivo studies, lipopolymer/FLT3 siRNA nanocomplexes were administered via tail-vein injection at a siRNA dose of 25 µg per mouse (~1 mg/kg) once the tumor volume was ≥50mm3 in AML MV4-11 subcutaneous xenograft models created in male NCG (ABL/VAF) mice. Four injections of the nanocomplexes were administered, each 72 hours apart with simultaneous measurements of the tumor volume. Gilteritinib was administered four times intraperitonially at a dose of 5 mg/kg, with a spacing of 72 hours between doses. A significant reduction in tumor volume was observed in the combination treated tumors compared to gilteritinib alone treated tumors. Refractory/relapsed tumors underwent a second round of treatment (with both nanocomplexes and gilteritinib being administered four times each). The combination therapy significantly increased median survival to 65 days as compared to 45 days with gilteritinib treatment alone (p<0.05). These findings demonstrate the promising potential of RNAi implemented with lipopolymer nanocomplexes for AML molecular therapy. The study prospectively supports the addition of RNAi therapy to current treatment modalities available to target the heterogeneity prevalent in AML.

Estimating the Lifetime Medical Cost Burden of an Allogeneic Hematopoietic Cell Transplantation Patient and the Value of Addressing the Unmet Need

Background: Allogeneic hematopoietic cell transplantation (allo-HCT) management has evolved, with the emergence of molecular detection assays and therapies to treat post-HCT complications such as graft-versus-host disease (GVHD). However, this approach could increase the economic burden on healthcare systems by only managing complications that occur after the initial transplant. Instead, alternative therapies involving novel engineered donor allografts to replace standard allo-HCT to prevent late complications may potentially improve outcomes and address a greater unmet need. Objective: To estimate (1) the average total lifetime medical costs of an allo-HCT and (2) to estimate the net monetary savings and value associated with reducing late complications. Methods: A short-term decision tree (100-day) and a long-term semi-Markov partitioned survival model were developed that estimated the average per-patient lifetime cost and expected quality-adjusted life years (QALYs) for an allo-HCT patient from a United States healthcare system perspective. The allo-HCT patient population represented an average 53-year-old patient who was undergoing an allo-HCT to treat the three most common indications for allo-HCT: acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), or myelodysplastic syndromes (MDS), distributed as 51%, 21%, and 28%, respectively. Key model clinical inputs included: overall survival, GVHD-free relapse-free survival (GRFS), incidence of both acute and chronic GVHD, relapse of the primary disease, and infections. Following the first 100 days, the patient cohort was assigned to 3 mutually exclusive health states in a semi-Markov partitioned survival model for the remainder of their lifetime: (1) GRFS, (2) progressed and/or GVHD, and (3) death. The projected time spent in each health state was derived from data provided by CIBMTR. Costs pertaining to allo-HCT and associated complications were obtained from the published literature based on medical claims analyses of payments (paid amounts) and were converted into costs using the American Hospital Association (AHA) commercial payment-to-cost ratio. Maintenance therapy costs for FLT3-ITD positive AML and Ph+ ALL cases were also included and were based on published literature, expert opinion, and prescribing information. The key outcomes were direct medical costs and expected QALYs. Cost results were reported as a range based on varying the percent of chronic GVHD patients that remained on treatment after two years (15% or 39%) as derived from the published literature. Scenario analyses were conducted to estimate the potential clinical and economic value that may be realized by improving GRFS outcomes. The scenario analyses to address some of the unmet need were modeled as a doubled improvement in GRFS (69% GRFS in year 1 as opposed to 34%) to estimate changes in allo-HCT-associated costs and survival. A willingness-to-pay threshold of $150,000 was used to monetize the QALYs gained as a result of improving GRFS outcomes. Results: Over a lifetime, the average per-patient medical cost of allo-HCT was estimated to range from $942,373 to $1,247,917. Of those total costs, 23% to 30% were incurred within the 1st 100 days and 42% to 56% within the 1st year. Of the medical costs estimated, the majority of the costs were for chronic GVHD treatment (37% to 53%) followed by the allo-HCT procedure (15% to 19%). Expected lifetime QALYs of an allo-HCT patient were estimated to be 4.7. In the scenario analyses, the unmet need that could be addressed by improving GRFS outcomes (via the medical cost offsets and QALYs gained) was associated with a net monetary value that ranged from $695,709 to $911,062. Of the estimated net monetary value, 85% to 88% was driven by direct medical cost savings while the remaining 12% to 15% was due to value of the gain in expected QALYs. Conclusions: The current per-patient cost of allo-HCT carries a substantial medical cost burden often exceeding $1M over the lifetime of the patient. Innovative research efforts focused on disease control and reduction or elimination of late complications altogether, particularly cGVHD, may provide the greatest value in improved patient outcomes and reduced healthcare spending. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Targeting FLT3 with a new-generation antibody-drug conjugate in combination with kinase inhibitors for treatment of AML

AbstractFms-like tyrosine kinase 3 (FLT3) is often overexpressed or constitutively activated by internal tandem duplication (ITD) and tyrosine kinase domain (TKD) mutations in acute myeloid leukemia (AML). Despite the use of receptor tyrosine kinase inhibitors (TKI) in FLT3-ITD–positive AML, the prognosis of patients is still poor, and further improvement of therapy is required. Targeting FLT3 independent of mutations by antibody-drug conjugates (ADCs) is a promising strategy for AML therapy. Here, we report the development and preclinical characterization of a novel FLT3-targeting ADC, 20D9-ADC, which was generated by applying the innovative P5 conjugation technology. In vitro, 20D9-ADC mediated potent cytotoxicity to Ba/F3 cells expressing transgenic FLT3 or FLT3-ITD, to AML cell lines, and to FLT3-ITD–positive patient-derived xenograft AML cells. In vivo, 20D9-ADC treatment led to a significant tumor reduction and even durable complete remission in AML xenograft models. Furthermore, 20D9-ADC demonstrated no severe hematotoxicity in in vitro colony formation assays using concentrations that were cytotoxic in AML cell line treatment. The combination of 20D9-ADC with the TKI midostaurin showed strong synergy in vitro and in vivo, leading to reduction of aggressive AML cells below the detection limit. Our data indicate that targeting FLT3 with an advanced new-generation ADC is a promising and potent antileukemic strategy, especially when combined with FLT3-TKI in FLT3-ITD–positive AML.

Abstract 3969: Y842 mutations in the activation loop of FLT3 regulates drug response in FLT3-ITD positive acute myeloid leukemia

Abstract Acute myeloid leukemia (AML) is an aggressive form of blood cancer with a poor prognosis. Approximately 30% of AML patients carry mutations in the type III receptor tyrosine kinase FLT3. The most common form of FLT3 mutations includes internal tandem duplication (ITD) mutations, which are also associated with poor clinical outcomes. Several inhibitors of FLT3 have been developed and are under clinical trials. Major problems in targeting FLT3-ITD include acquired resistance to the inhibitors which is mediated by secondary point mutations in FLT3. The activity of many, but not all, tyrosine kinases is dependent on the well-conserved tyrosine residue in the activation loop for receptor activation. For instance, we have recently shown that the KIT receptor is dependent on its activation loop tyrosine for downstream signaling and receptor stability, but not for the activation of the receptor. In many AML patients, FLT3-ITD mutations in combination with mutations in the activation loop tyrosine residue (Y842) result in drug resistance. We therefore stably transduced cells with FLT3-ITD in combination with different Y842 mutants (Y842C and Y842F). Using MM-PBSA, we found that the introduction of the Y842C and Y842F point mutations changed the binding energy of the FLT3 inhibitors quizartinib, sorafenib, and midostaurin in silico. In addition, the mutants displayed a change in drug response and apoptosis levels after treatments with FLT3 inhibitors in vitro. Previous data from our lab also shows that the Y842F mutant resulted in delayed tumor formation in vivo, indicating that the phosphorylation of the activation loop tyrosine is an important event in FLT3-ITD-mediated transformation. All in all, our data suggest that the activation loop tyrosine residue in FLT3-ITD positive cells plays an important role in drug response and tumor formation. Citation Format: Lina Al Ashiri, Sausan Moharram, Rituraj Purohit, Julhash Kazi Uddin, Lars Rönnstrand. Y842 mutations in the activation loop of FLT3 regulates drug response in FLT3-ITD positive acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3969.

Abstract 3269: Dual targeting of FLT3 and LSD1 disrupts the MYC super-enhancer complex in acute myeloid leukemia

Abstract Clinical responses to kinase inhibitor therapy in acute myeloid leukemia (AML) are limited by development of resistance. A major contributor of resistance is epigenetic adaptation to kinase inhibitor therapy. We present evidence that inhibition of the epigenetic regulator lysine-specific demethylase 1 (LSD1) augments the response to inhibitors of FMS-like tyrosine kinase 3 (FLT3) in FLT3-mutant AML. We demonstrate that combined FLT3 and LSD1 inhibition results in synergistic cell death of FLT3-mutant AML cell lines and primary patient samples. High-resolution epigenetic sequencing revealed that the combination therapy synergistically suppresses pro-proliferative MYC-bound promoters and activates pro-differentiative PU.1-bound enhancers. Regulon enrichment analysis in primary AML samples nominated STAT5 (a downstream target of activated FLT3 signaling) as a putative regulator of MYC gene expression. STAT5 is highly bound to the MYC blood super-enhancer and FLT3 inhibition results in a loss of both STAT5 binding and MYC blood super-enhancer chromatin accessibility. In contrast, LSD1 inhibition suppresses MYC target genes by accumulation of repressive H3K9me1 marks. We validated these findings in 72 primary AML samples, including 19 FLT3-ITD positive AML samples. The combination improved responses in the vast majority of patient samples, and high MYC regulon activity was a predictor of response. Gene expression profiling in treated primary AML samples confirmed that dual FLT3 and LSD1 inhibition activates PU.1 target genes and suppresses MYC target genes. Finally, single cell ATAC-seq on primary AML blasts treated ex vivo with combined FLT3 and LSD1 shifted cells from a MYC super-enhancer high to MYC super-enhancer low state. Collectively, these data provide preclinical rationale for the investigation of dual FLT3 and LSD1 inhibition in clinical trial. Citation Format: William M. Yashar, Brittany M. Smith, Jake VanCampen, Garth L. Kong, Jommel Macaraeg, Daniel J. Coleman, Brian J. Druker, Julia E. Maxson, Theodore P. Braun. Dual targeting of FLT3 and LSD1 disrupts the MYC super-enhancer complex in acute myeloid leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3269.

Preclinical and Pilot Study of Type I FLT3 Tyrosine Kinase Inhibitor, Crenolanib, with Sorafenib in Acute Myeloid Leukemia and FLT3-Internal Tandem Duplication.

To evaluate the safety, activity, and emergence of FLT3-kinase domain (KD) mutations with combination therapy of crenolanib and sorafenib in acute myeloid leukemia (AML) with FLT3-internal tandem duplication (ITD). After in vitro and xenograft efficacy studies using AML cell lines that have FLT3-ITD with or without FLT3-KD mutation, a pilot study was performed with crenolanib (67 mg/m2/dose, three times per day on days 1-28) and two dose levels of sorafenib (150 and 200 mg/m2/day on days 8-28) in 9 pediatric patients with refractory/relapsed FLT3-ITD-positive AML. Pharmacokinetic, pharmacodynamic, and FLT3-KD mutation analysis were done in both preclinical and clinical studies. The combination of crenolanib and sorafenib in preclinical models showed synergy without affecting pharmacokinetics of each agent, inhibited p-STAT5 and p-ERK for up to 8 hours, and led to significantly better leukemia response (P < 0.005) and survival (P < 0.05) compared with single agents. Fewer FLT3-KD mutations emerged with dose-intensive crenolanib (twice daily) and low-intensity sorafenib (three times/week) compared with daily crenolanib or sorafenib (P < 0.05). The crenolanib and sorafenib combination was tolerable without dose-limiting toxicities, and three complete remissions (one with incomplete count recovery) and one partial remission were observed in 8 evaluable patients. Median crenolanib apparent clearance showed a nonsignificant decrease during treatment (45.0, 40.5, and 20.3 L/hour/m2 on days 1, 7, and 14, respectively) without drug-drug interaction. Only 1 patient developed a FLT3-KD mutation (FLT3 F691L). The combination of crenolanib and sorafenib was tolerable with antileukemic activities and rare emergence of FLT3-TKD mutations, which warrants further investigation.

Phosphorylation of SHP2 at Tyr62 Enables Acquired Resistance to SHP2 Allosteric Inhibitors in FLT3-ITD-Driven AML.

These findings suggest that combined inhibition of SHP2 and FLT3 effectively treat FLT3-ITD-positive AML, highlighting the need for development of more potent SHP2 inhibitors and combination therapies for clinical applications.

KDM6 Demethylases Integrate DNA Repair Gene Regulation: Loss of KDM6A Sensitizes AML to PARP Inhibition and Potentiates with BCL2 Blockade

Acute myeloid leukemia (AML) is a heterogeneous, aggressive hematological malignancy with dismal prognosis where limited targeted therapies are currently available. Poly-(ADP-ribose)-polymerase (PARP) inhibition has emerged as an important therapeutic arsenal to target homologous recombination-deficient tumors. However, molecular understanding of PARP blockade in the context of epigenetic derangements and transcriptional plasticity in human elderly AML pathogenesis remains unexplored. KDM6 proteins are H3K27 demethylases that critically regulate chromatin architecture in multi-cellularity and tumorigenesis (Tran, Mol Cell Biol 2020). KDM6A escapes X-chr inactivation, and Utx-/- female mice spontaneously develop aging associated myeloid leukemia (Gozdecka, Nat Genet 2018; Sera, Blood 2021). In addition, KDM6A loss of function mutation is implicated in AML relapse (Stief, Leukemia 2020). In contrast, KDM6B primarily exerts an oncogenic function in heme-malignancies. Together, KDM6A and KDM6B play cell type-specific function in leukemia, and KDM6 proteins and associated signaling emerge as important focal point for developing molecular targeted therapy.We identify that KDM6 demethylase activity critically regulates DNA damage repair (DDR) gene expression program in AML. Transcriptome analysis indicated a significant downregulation of expression of DDR genesets in both KDM6A deficient human AML and Utx -/- pre-leukemic cells. Lentiviral shRNA screening performed in response to low-dose γ-irradiation in AML stem cells, revealed a radioprotective function of KDM6A. Expression of KDM6s is regulated by genotoxic stress in a time-dependent manner, and deficiency of JmjC catalytic function impaired DDR transcriptional activation and compromised repair potential. Mechanistically, quantitative ChIP experiments also revealed co-operation between KDM6A and SWI/SNF facilitating dynamic chromatin remodeling at TSS/promoter to induce DDR gene expression. To interrogate changes in chromatin accessibility we performed ATAC-seq analysis in KDM6 deficient AML. Motif enrichment highlighted that while KDM6A depletion led to reduced chromatin access to 140 transcription factors (TFs), only 56 TF binding sites showed increased accessibility. Overall, changes in chromatin accessibility, associated with a reduced binding of DDR regulatory TFs in KDM6 deficient AML, account for a compromised DDR function.In agreement with these findings an array of KDM6 deficient AML cells were more sensitive to PARP inhibition, and pre-clinical mice models xenotransplanted with KDM6A loss of function AML line showed an increased susceptibility to PARP blockade in vivo. FLT3-ITD positive AML with a lower KDM6A expression was more sensitive to olaparib. In addition, olaparib administration significantly reduced bone marrow engraftment of patient-derived xenografts of KDM6A-mutant primary AML. Interestingly, KDM6A expression is upregulated in venetoclax-resistant monocytic-AML compared to venet-sensitive primitive-AML. Using venet responsive isogenic lines we demonstrated that attenuation of KDM6 function increased mitochondrial activity, intracellular ROS levels, de-repressed BCL2 expression, and sensitized AML cells to venetoclax. Additionally, KDM6 loss resulted in transcriptional repression of BCL2A1, commonly associated with venet resistance (Zhang, Nat Cancer 2020). Corroborating these results, dual targeting of PARP with BCL2 was superior to PARP or BCL2 inhibitor monotherapy in inducing primary AML apoptosis, and KDM6A loss further enhanced this synergism.In sum, our study illustrates a molecular mechanistic rationale in support for a novel combination targeted therapy for AML, and posit KDM6A as a molecular determinant for therapeutic efficacy. Intriguingly, KDM6A functions as a gatekeeper of BCL2 and BCL2A1 expression. Similar to TET2 although bi-allelic Utx loss causes evolution to myeloid neoplasms, minimal KDM6 activity is important for survival of human AML cells. KDM6s have been implicated in solid tumors, and both PARP and BCL2 inhibitors are being tested in cancer patients, underscoring a wider scope of application. To conclude, KDM6A unfolds to be a central regulator for susceptibility of AML to both PARP and BCL2 inhibition, expanding the possibility to characterize effective combination targeted therapy for AML in clinical settings. DisclosuresMinden: Astellas: Consultancy. Dick: Celgene, Trillium Therapeutics: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Abrogation of a Histone Chaperone Pathway Mitigates Inflammation-Driven AML Progression

Background: Genetic heterogeneity makes clinical interventions challenging for acute myeloid leukemia (AML) patients. Identifying and targeting microenvironment-driven pathways that are active across AML genetic subtypes should allow the development of more broadly effective therapies. Previously, we have shown that AML microenvironment is rich in proinflammatory cytokine interleukin-1β (IL-1β) and significantly promotes the growth of AML progenitors while suppressing healthy progenitors. To elucidate this paradoxical effect, we performed transcriptome (RNA-seq) analysis from IL-1β-stimulated CD34+ AML and normal progenitors and found that ASF1B (anti-silencing function-1B) is one of the most differentially expressed genes. ASF1B is a histone chaperone, which recruits H3-H4 histones onto the replication fork during S-phase. This process is regulated by tousled-like kinase 1 and 2 (TLKs). TLKs and ASF1B are overexpressed in multiple solid tumors and associated with poor prognosis. However, their functional roles in hematopoiesis and inflammation-driven leukemia are unexplored. Here, we reveal a novel molecular mechanism that IL-1β promotes leukemia progression by activating the TLK-ASF1B pathway.Methods and Results: We first confirmed that IL-1β stimulation upregulates ASF1B expression at both mRNA and protein levels in FLT3-ITD, MLL-ENL, and NPM1 positive primary AML samples. ASF1B upregulation is abolished upon treatment with a p38MAPK inhibitor, further suggesting that ASF1B is downstream of IL-1β/p38 signaling. Next, we stably knocked down ASF1B in an AML cell line MOLM-14 using doxycycline-inducible shRNA. ASF1B-depleted AML cells exhibited reduced cell viability, inhibited growth, blocked cell cycle progression, and impaired colony formation ability. We xenografted shASF1B expressing AML cells into NSG mice and induced knockdown in vivo. Flow cytometry analysis of bone marrow cells 3 weeks post-engraftment showed 80 percent reduction in leukemia burden following ASF1B silencing compared to controls.To determine whether upregulation of ASF1B contributes to IL-1β-driven leukemic growth, we overexpressed ASF1B with MLL-ENL oncogene in a murine bone marrow transplantation model. We found that daily IL-1β exposure accelerated leukemia progression compared to vehicle-treated group (median survival = 64 vs. 85 days, p<0.05), and this effect was phenocopied by overexpression of ASF1B (median survival = 62 vs. 85 days, p<0.05). Conversely, heterozygous and complete Asf1b deletion in the MLL-ENL AML model delayed the leukemia progression compared to wildtype mice. Furthermore, Asf1b deletion attenuated IL-1β-mediated AML progression compared to wildtype controls (median survival = 63 vs. 47 days, p <0.01). Immunophenotyping of Asf1b-deficient mice using flow cytometry suggested that ASF1B is dispensable for normal hematopoiesis. Together, these data suggested that targeting of the ASF1B pathway may spare healthy cells. Additionally, we found that TLK2 which regulates ASF1B activation, is also upregulated in AML progenitors compared to healthy cells at the baseline levels and upon IL-1β stimulation. We next knocked down TLKs in human AML cells and observed a similar pattern with growth arrest, higher replication stress and DNA damage response. Finally, we generated Vav-Cre+ Tlk2 mice allowing Tlk2 deletion only in hematopoietic cells and found that Tlk2 deletion prolongs survival of leukemic mice in a dose dependent manner with and without IL-1β stimulation.Conclusions: We demonstrate that increased TLK-ASF1B expression promotes survival of AML cells. We also provide the first in vitro and in vivo evidence that the TLK-ASF1B pathway plays a critical role in potentiating IL-1β-dependent AML growth. Therefore, we establish TLK-ASF1B pathway as a novel route for therapeutic strategy to suppress inflammation-driven growth in various AML genetic subtypes. DisclosuresNo relevant conflicts of interest to declare.

Development of Anti-CD99scFvs for the Treatment of Acute Myeloid Leukemia

CD99 has gained much attention in recent years as a novel therapeutic target in hematological malignancies, due to its upregulation in acute myeloid leukemia (AML) and acute lymphoid leukemia (ALL). We have recently shown that targeting CD99 with a knockdown approach or with commercial antibodies results in antileukemia activity in AML cells. We have also developed anti-CD99 nanoparticles and demonstrated excellent in vitro and in vivo antileukemia activity. Here we report the development of human single chain variable fragment targeting CD99 (anti-CD99 scFv) and the preclinical activity in AML cells and in AML xenograft mouse model. The anti-CD99 scFv was developed by inserting CD99scFv DNA sequence into the pFUSE vector, encoding pATL103-CD99 scFvs. To produce anti-CD99 scFv, the plasmids were transfected in Expi293 (HEK293T) cells. Then the medium was harvested and underwent two cycles of dialysis. To determine product purity, proteins were analyzed on SDS-PAGE gel stained with Coomassie blue. For each 240ml cell culture media with recombinant CD99 plasmids, we obtained about 1-2mg anti-CD99 scFv. The binding affinity of CD99 scFv to CD99 surface protein was assessed in 293T cells (CD99 null cells) which exhibited no binding and in MOLM-13 cells and MV4-11 (CD99 positive AML cells) which demonstrated strong binding. Treatment with 5uM of anti-CD99 scFv significantly reduced cell viability in both leukemic cell line MOLM-13 and MV4-11 (MOLM-13: 35.14%, P= 0.008; MV4-11: 30.17%, P=0.002) and primary AML patient blasts (29.37%, P=0.048) compared with control cells. Colony forming assay showed that anti-CD99 scFv treated AML blasts exhibited less number of colonies compared with control cells (plating efficiency (PE): 0.035% vs 0.12%). We also established the in vivo antileukemia activity of anti-CD99 scFv using MOLM-13 cells (FLT3-ITD positive AML cells) NOD scid gamma (NSG) xenograft mouse model. MOLM-13 (2.5x10^6) cells were engrafted into NSG mice via IV tail injection (N=4 mice per group). Mice were treated with PBS (group 1) or 4mg/kg of anti-CD99 scFV on days 10, 14, 18 and 22 post cell engraftment. Mice were euthanized on day 24 and levels of leukemia engraftment were assessed by flow cytometry measurements of huCD45 staining of cells collected from the bone marrow and the peripheral blood and compared between the two groups. Mice treated with four doses of 4mg/kg CD99scFv demonstrated significant reduction in leukemia engraftment in the bone marrow assessed by flow cytometry measurements of huCD45 staining compared with the PBS mice group (huCD45%: 39.7 vs 56, P = 0.0017). In conclusion, we report the development of anti-CD99 single chain variable fragments for the treatment of AML. Our study demonstrates good binding affinity and specificity and a promising preclinical antileukemia activity both in AML cells and in xenograft mouse model. DisclosuresYaghmour: Jazz: Consultancy, Honoraria; Astellas: Consultancy; Takeda: Consultancy; Incyte: Consultancy.

"Combined Flow Cytometry and Molecular Monitoring of Central Nervous System Relapse in a Patient with FLT3-ITD and NPM1 Positive AML"

We here describe a 58 years-old male patient diagnosed in September 2019 with acute myeloid leukemia (AML) in another hematological center and referred to us to receive allogeneic stem cell transplantation...

SETBP1 overexpression acts in the place of class-defining mutations to drive FLT3-ITD–mutant AML

Advances in cancer genomics have revealed genomic classes of acute myeloid leukemia (AML) characterized by class-defining mutations, such as chimeric fusion genes or in genes such as NPM1, MLL, and CEBPA. These class-defining mutations frequently synergize with internal tandem duplications in FLT3 (FLT3-ITDs) to drive leukemogenesis. However, ∼20% of FLT3-ITD–positive AMLs bare no class-defining mutations, and mechanisms of leukemic transformation in these cases are unknown. To identify pathways that drive FLT3-ITD mutant AML in the absence of class-defining mutations, we performed an insertional mutagenesis (IM) screening in Flt3-ITD mice, using Sleeping Beauty transposons. All mice developed acute leukemia (predominantly AML) after a median of 73 days. Analysis of transposon insertions in 38 samples from Flt3-ITD/IM leukemic mice identified recurrent integrations at 22 loci, including Setbp1 (20/38), Ets1 (11/38), Ash1l (8/38), Notch1 (8/38), Erg (7/38), and Runx1 (5/38). Insertions at Setbp1 led exclusively to AML and activated a transcriptional program similar, but not identical, to those of NPM1-mutant and MLL-rearranged AMLs. Guide RNA targeting of Setbp1 was highly detrimental to Flt3ITD/+/Setbp1IM+, but not to Flt3ITD/+/Npm1cA/+, AMLs. Also, analysis of RNA-sequencing data from hundreds of human AMLs revealed that SETBP1 expression is significantly higher in FLT3-ITD AMLs lacking class-defining mutations. These findings propose that SETBP1 overexpression collaborates with FLT3-ITD to drive a subtype of human AML. To identify genetic vulnerabilities of these AMLs, we performed genome-wide CRISPR-Cas9 screening in Flt3ITD/+/Setbp1IM+ AMLs and identified potential therapeutic targets, including Kdm1a, Brd3, Ezh2, and Hmgcr. Our study gives new insights into epigenetic pathways that can drive AMLs lacking class-defining mutations and proposes therapeutic approaches against such cases.