FLT3 Internal Tandem Duplication Research Articles

The FLT3 F691L Gatekeeper Mutation Promotes Clinical Resistance to Gilteritinib + Venetoclax (GILT + VEN) in AML

Background: FMS-like tyrosine kinase (FLT3) is one the most frequently mutated genes in AML and is associated with poor prognosis. FLT3 internal tandem duplication (ITD) and tyrosine kinase domain (TKD) mutations occur in up to 30% and 5-10% of AML, respectively. Several small molecule FLT3 inhibitors (FLT3i) have been developed but their use as single agents is limited due to the development of drug resistance. Our lab developed a two-step model of early and late resistance to FLT3i that recapitulates resistance in AML patients (Traer et al. Cancer Res. 2016; Joshi et al. Cancer Cell 2021). Early resistance, also known as AML persistence, is the stage when residual AML cells are dependent upon the marrow microenvironment for survival and patients are clinically responding. Late resistance to GILT was characterized by expansion of intrinsic mutations, with NRAS mutations being the most frequent mutation, in addition to a few gatekeeper FLT3 mutations. Current therapies are looking at combinations to overcome GILT resistance, including chemotherapy, hypomethylating agents (HMAs), and venetoclax (VEN) +/- HMAs. GILT+VEN, in particular, has shown good initial activity in relapsed/refractory FLT3 AML patients (Daver et al. ASH 2020), however the mechanism of resistance to this combination is unknown.Results: Early resistance cell cultures to GILT+VEN were created by exposing MOLM14 cells to GILT 25nM + VEN 25nM alone or supplemented with microenvironmental ligands FGF2 or FLT3 ligand (FL; N=3/group). Media, drugs, and ligands were replenished twice weekly. After 25 weeks, only the cultures exposed to ligand resumed growth (N=1 for FGF2 and N=3 for FL). Ligands were then removed from these early resistant cultures to induce late resistance. There was an initial drop in cell viability but cells resumed growth after only 3.5 weeks (Fig. 1). In contrast, the time to develop early and late resistance to GILT monotherapy was 8 and 15 weeks, respectively. Immunoblot analysis of GILT + VEN early and late resistant cultures demonstrated restoration of FLT3 signaling, as well as phosphorylation of downstream AKT/MAPK pathways. These results also contrasted to late GILT monotherapy resistant cultures, which had downstream AKT/MAPK activation via outgrowth of NRAS mutations. Since FLT3 appeared to be functionally active, we sequenced FLT3 and found that all early and late GILT + VEN resistance cultures had gatekeeper FLT3 F691L mutations. F691L accounted for only in a minority of resistance cultures to GILT monotherapy. To test if FLT3 signaling was important for resistance, we exposed parental cells to higher concentrations of gilteritinib, which have been shown to partly overcome F691L, as well as the FLT3i FF-10101, which binds FLT3 at a different site and is not affected by the F691L mutation. Both of these approaches restored sensitivity to FLT3i in vitro. As expected, the F691L mutation provided broad resistance to most FLT3i (Fig. 2).To validate this mechanism of resistance in patients, we identified a relapsed FLT3-ITD patient who was treated with GILT monotherapy for 5 months, followed by GILT + HMA for 4 cycles, and then GILT + VEN for resistant proliferative disease. After an initial response to GILT + VEN, the leukemia cells began to increase again in the peripheral blood. A repeat genetic test was ordered and the patient was found to have developed a FLT3 F691L mutation at a high variant allele frequency (Fig. 3).Conclusion: We have developed a robust cell line model of early and late resistance to FLT3i that mimics the timing and expansion of resistance mutations in the clinic. Our model of early and late resistance to GILT combinations can prospectively predict mechanisms of resistance. Although uncommon as a mechanism of resistance to GILT monotherapy, our model and early patient data predicts that F691L mutations are more important for GILT + VEN resistance. [Display omitted] DisclosuresTyner: Seattle Genetics: Research Funding; Astrazeneca: Research Funding; Array: Research Funding; Janssen: Research Funding; Takeda: Research Funding; Gilead: Research Funding; Incyte: Research Funding; Petra: Research Funding; Constellation: Research Funding; Genentech: Research Funding; Agios: Research Funding; Schrodinger: Research Funding. Traer: ImmunoGen: Membership on an entity's Board of Directors or advisory committees; Schrodinger: Research Funding; Genentech: Membership on an entity's Board of Directors or advisory committees; Servier/Agios: Membership on an entity's Board of Directors or advisory committees; Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees; Incyte: Research Funding; Astellas: Consultancy, Membership on an entity's Board of Directors or advisory committees.

FLT3 ITD and NUP98 translocations Cooperate to Induce Type I Interferon Signaling That Impedes Hematopoietic Progenitor Differentiation

FLT3-Internal Tandem Duplication (FLT3 ITD) mutations occur at similar frequencies in both childhood and adult acute myeloid leukemia (AML) patients, but they cooperate with different initiating mutations at different ages. In childhood AML, FLT3 ITD often cooperates with translocations that generate fusion proteins, such as NUP98 translocations (NUP98-t). In adult AML, FLT3 ITD often cooperates with mutations that inactivate RUNX1, DNMT3A, TET2 and other genes that regulate self-renewal and myeloid differentiation. This raises the question of whether FLT3 ITD induces similar, or distinct, changes in gene expression and cell fate when it arises in the setting of pediatric and adult driver mutations. To answer this question, we evaluated pre-leukemic changes in hematopoiesis and gene expression in neonatal and juvenile mice that carried pediatric-like FLT3 ITD/NUP98-HOXD13 and adult-like FLT3 ITD/Runx1 DEL/+ mutation profiles. Both NUP98-HOXD13 (NHD13) and Runx1 DEL/+ severely depleted phenotypic hematopoietic stem cells (HSCs) when coupled with FLT3 ITD. However, FLT3 ITD/NHD13 cooperated to enhancer ectopic self-renewal capacity in lineage committed hematopoietic progenitor cells (HPCs) whereas FLT3 ITD/Runx1 DEL/+ did not. FLT3 ITD/NHD13 and FLT3 ITD/Runx1 DEL/+ both induced changes in gene expression and enhancer activation that were not seen with either mutation alone. These cooperative changes were almost entirely non-overlapping. The most notable cooperative change in FLT3 ITD/NHD13 HPCs was activation of a type I Interferon target gene signature. By contrast, this signature was actively repressed in FLT3 ITD/Runx1 DEL/+ HPCs due to mir155 activation. Thus, FLT3 ITD/NHD13 and FLT3 ITD/Runx1 DEL/+ mutations have largely non-overlapping, cooperative effects on hematopoietic cell fate and gene expression.Interferon signaling sustained the FLT3 ITD/NHD13 HPC population by impeding differentiation into more committed myeloid progenitors. When we deleted the type I interferon receptor (Ifnar) on a FLT3 ITD/NHD13 background, the HPC population was virtually eliminated, yet the pre-granulocyte monocyte (pGM) and granulocyte-monocyte progenitor (GMP) populations expanded. HPC proliferation and death did not change in the absence of Ifnar. These findings suggest that type I interferon impedes HPC differentiation into pGMs during the neonatal and juvenile stages of life. Of note, ectopic interferon was insufficient to enhancer HPC numbers in setting of a sole NHD13 mutation. Thus, interferon is necessary to sustain FLT3 ITD/NHD13 HPCs, but not sufficient, and other signaling pathways downstream of FLT3 ITD must cooperate with NHD13 in parallel to drive HPC expansion. To our surprise, Ifnar deletion did not reduce repopulating activity in FLT3 ITD/NHD13 bone marrow - as measured by limiting dilution transplants - it simply shifted repopulating activity to more committed pGMs. Our raise two important conceptual points. First, we show that different initiating mutations can elicit completely different transcriptional responses downstream of FLT3 ITD. Precision medicine strategies must account for these differences as effective therapies for adult FLT3 ITD AML may not yield similar success in the pediatric population. Second, type I interferon shapes pre-leukemic hematopoiesis in the setting of FLT3 ITD/NUP98-t mutations. In doing so, the mutant cells may hijack a program that we recently identified as promoting normal perinatal HPC expansion. DisclosuresNo relevant conflicts of interest to declare.

A Phase 3, Randomized, Double-Blind, Placebo-Controlled Study to Assess the Efficacy and Safety of Entospletinib in Combination with Intensive Induction and Consolidation Chemotherapy in Adults with Newly Diagnosed Nucleophosmin 1-mutated Acute Myeloid Leukemia

A Phase 3, Randomized, Double-Blind, Placebo-Controlled Study to Assess the Efficacy and Safety of Entospletinib in Combination with Intensive Induction and Consolidation Chemotherapy in Adults with Newly Diagnosed Nucleophosmin 1-mutated Acute Myeloid Leukemia

A Systematic Review and Meta-Analysis Comparing Type I and II FLT3 Inhibitors in Relapsed/ Refractory Acute Myeloid Leukemia and High-Risk Myelodysplastic Syndrome

Background: Acute myeloid leukemia (AML) is frequently characterized by mutations in FMS-like tyrosine kinase 3 (FLT3). FLT3 is mutated in one-third of de novo AML patients and recurrent in relapsed/refractory (R/R) AML. More frequent FLT3 internal tandem duplications (ITD) and less frequent tyrosine kinase domain (TKD) point mutations are prognostically important in AML and are associated with an inferior outcome. Although targeted therapy with FLT3 inhibitors improves prognosis in FLT3 mutated AML patients, there is a lack of evidence regarding the superiority of efficacy of various FLT3 inhibitors in clinical practice and the therapeutic pipeline. Broadly grouped as type 1 (targets ITD and TKD, e.g., midostaurin) and type 2 (targets only ITD, e.g., quizartinib) inhibitors, there is no consolidated summary of the difference in efficacy between these two classes of FLT3 inhibitors in R/R AML and high-risk myelodysplastic syndrome (HR-MDS). Hence, we conducted a systematic review and meta-analysis of various type 1 and 2 FLT3 inhibitors tested in clinical trials to compare their clinical effectiveness in treating R/R AML and HR-MDS.Methods: PubMed and EMBASE databases were searched for clinical trials published between 1/1/2000 and 5/26/2021 using a combination of keywords and subject headings related to FLT3 inhibitors and AML. Two independent reviewers screened titles/abstracts and full texts, with a third reviewer resolving conflicts. Studies were included if they were (1) full-length published journal articles that (2) reported the results of single-arm or double-arm phase I/II/III clinical trials in patients with R/R AML or HR-MDS. Outcomes of interest were composite response rate (CRc=complete response + complete response with incomplete count recovery) and overall response rate (ORR).Results: Record screening resulted in the initial inclusion of 30 studies for qualitative analysis (Fig. 1). Due to poor representation of R/R AML patients and post-transplant FLT3 inhibitor outcomes, two studies were excluded from quantitative analysis, leaving 28 studies for the final analysis. The studies had a total of 1927 R/R AML and HR-MDS patients with a median age of 61.5 years (range, 13-91) and a median of three (range, 0-10) prior lines of treatment. Giltertinib (n=3 studies) was the most frequently studied type 1 inhibitor, and quizartinib (n=6) and sorafenib (n=5) were the most frequently studied type 2 inhibitors. Heterogeneity testing using Cochran's Q test and I 2 values showed the presence of heterogeneity for both CRc (p<0.01 and I 2=87% for both type 1 and 2) and ORR (p<0.01 for both type 1 and 2, I 2=88% and 92% for type 1 and type 2, respectively) and. Hence, random-effects models were used. Asymmetry tests performed using Egger's linear regression test suggested publication bias only for type 2 (p=0.005) for ORR and type 1 and 2 for CRc (p=0.017 and 0.002, respectively). Thus, the trim-and-fill method was used to adjust for publication bias for type 2 for ORR and type 1 and 2 for CRc. Pooled ORR was 37% (95% CI, 25-51%) for type 1 and 58% (95% CI, 43-71%) for type 2. The highest ORR was for gilteritinib (68%; 95% CI, 61-73%) among type 1 and sorafenib (99%; 95% CI, 81-100%), linifanib (99%; 95% CI, 89-100%), and tandutinib (100%; 95% CI, 93-100%) for type 2 (Fig. 2). Pooled CRc was 35% (95% CI, 21-52%) for type 1 and 38% (95% CI, 27-50%) for type 2. The highest CRc was for midostaurin (98%; 95% CI, 79-100%) among type 1 and sorafenib (97%; 95% CI, 68-100%), tandutinib (98%; 95% CI, 72-100%), and semaxinib (98%; 78-100%) among type 3 (Fig. 3).Conclusion: Both type 1 and 2 inhibitors are effective as monotherapy in R/R AML and HR-MDS patients. Though not statistically significant, better ORR for type 2 than for type 1 inhibitors in the R/R setting provides a background to explore the biological heterogeneity of response and preferential sensitivity to ITD-targeted therapy alone rather than the dual inhibitor. More prospective randomized study designs comparing type 1 and 2 inhibitors in the R/R and de novo setting are needed. [Display omitted] DisclosuresMaciejewski: Bristol Myers Squibb/Celgene: Consultancy; Regeneron: Consultancy; Alexion: Consultancy; Novartis: Consultancy. Balasubramanian: Servier Pharmaceuticals: Research Funding.

TP-0184, a Novel FLT3-ALK2 Dual Inhibitor, Targets Amino Acid Transport and Biosynthesis in AML Cells and Sensitizes AML Cells to Chemotherapy and BCL2 Inhibition

Background FMS-like tyrosine kinase 3 (FLT3), a transmembrane receptor tyrosine kinase that is frequently mutated in AML, is associated with poor prognosis. Inhibitors of FLT3 internal tandem duplication (ITD) mutants and wild-type (WT) FLT3 have been studied, but their clinical usefulness is limited owing to treatment resistance. However, the molecular factors contributing to this resistance are unknown. We reported that AML cells induce osteogenic differentiation of bone marrow-derived mesenchymal stromal cells through the bone morphogenetic protein (BMP)-mediated signaling pathway, promoting leukemic growth. However, the effects of targeting BMP signaling in patients with FLT3-mutated AML are unexplored. Here, we hypothesized that the BMP type 1 receptor ALK2 is a key biomarker and a therapeutic target in AML patients with FLT3-ITD mutations and that the FLT3-ALK2 dual inhibitor TP-0184 inhibits leukemia growth.Methods: To determine whether ALK2 is a potential target in AML patients with FLT3-ITD mutations, we analyzed gene expression datasets (OHSU and TCGA). We treated 9 AML cell lines with FLT3-WT or ITD mutations with varying doses of ALK2 inhibitors (LDN-212854 or TP-0184) and measured their effect on cell proliferation and the cell cycle. To determine the mechanism of TP-0184-mediated cell cycle arrest, we measured activation of FLT3 downstream signaling by using Western blotting and RNA sequencing. IncuCyte live-cell imaging was used to determine the apoptotic effects of TP-0184 in combination with chemotherapy or targeted therapy agents. Further, we performed human RTK kinase binding assay to understand the binding specificity of TP-0184 with 11 different FLT3 mutants. Finally, the effect of TP-0184 on AML growth in vivo was investigated using an FLT3-ITD positive AML xenograft model (MOLM13).Results: Analysis of AML datasets showed that ALK2 is significantly upregulated in AML patients with FLT3 mutations compared to those with WT-FLT3 (p < 0.00001) and predicts poor overall survival (p = 0.05). Validating these findings, we found higher ALK2 mRNA expression in AML cell lines with FLT3-ITD mutations than in those with WT-FLT3 (p = 0.039). This suggests that ALK2 could serve as a therapeutic target in AML with FLT3-ITD mutation. Treatment of FLT3-WT and -mutated AML cell lines with the ALK2 inhibitors LDN-212854 and TP-0184 resulted in significant inhibition of FLT3-ITD-mutant cell growth at low concentrations (IC50<25nM), while WT-FLT3 cells were affected only at high concentrations (IC50>100nM). Interestingly, TP-0184 was 10-fold more potent in inhibiting AML cell proliferation than was LDN-212854. TP-0184 induced G1/G0 arrest in AML cell lines with FLT-ITD mutations but had minimal to no effect in FLT3-WT AML cells, suggesting that AML cell lines with FLT3-ITD mutations depend on ALK2 for their survival. Further, we observed that treatment with TP-0184 in AML cell lines significantly inhibited multiple signaling proteins downstream of FLT3, such as p-STAT5, p-MKK3, and p-ERK, as well as p-PI3K, p-AKT, p-mTOR, p-4E-BP1, and p-S6K. Gene expression analysis revealed that treatment with TP-0184 in FLT3-ITD cell lines significantly downregulated the serine biosynthesis pathway, which is essential in these cells (Bjelosevic S. et al., Cancer Discov, 2021). Moreover, molecular docking and kinase-binding studies revealed that TP-0184 is bound to wild-type FLT3 as well as most of the FLT3 mutants with dissociation constants (KD) less than 5nM. These data suggest that TP-0184 inhibits both mutant FLT3 and ALK2 in AML cells. Interestingly, TP-0184 plus chemotherapy showed a synergistic effect only in FLT3-ITD cell lines, whereas TP-0184 plus the BCL2 inhibitor, venetoclax was synergistic in both FLT3-ITD and FLT3-WT cell lines. Lastly, treatment with TP-0184 inhibited AML growth and significantly prolonged survival of FLT3-ITD-mutated AML-bearing mice in a dose-dependent manner (p <0.0001).Conclusion: Our data indicate that ALK2 is a prognostic marker for AML patients with FLT3-ITD mutations. TP-0184 significantly inhibits cell proliferation by inhibiting signaling pathways downstream of FLT3, including serine biosynthesis, in AML cells. Kinase assays confirmed that TP-0184 is a highly specific FLT3 inhibitor as well as an ALK2 inhibitor. TP-0184 sensitizes AML cells to chemotherapeutic agents and targeted therapy and inhibits AML growth in vivo. DisclosuresFoulks: Sumitomo Dainippon Pharma Oncology: Patents & Royalties: WO2021102343A1; Sumitomo Dainippon Pharma Oncology: Patents & Royalties: CA3103995A1; Sumitomo Dainippon Pharma Oncology: Patents & Royalties: US11040038B2. Warner: Sumitomo Dainippon Pharma Oncology: Patents & Royalties: US11040038B2; Sumitomo Dainippon Pharma Oncology: Patents & Royalties: US10752594B2; Sumitomo Dainippon Pharma Oncology: Patents & Royalties: CA3103995A1; Sumitomo Dainippon Pharma Oncology: Patents & Royalties: WO2021102343A1. Battula: Tolero Pharmaceuticals: Research Funding.

Mannose Metabolism Is a Metabolic Vulnerability Unveiled By Standard and Novel Therapies in Acute Myeloid Leukemia

IntroductionThe development of resistance to standard and novel therapies remains the main obstacle to cure in acute myeloid leukemia (AML). Metabolic rewiring has emerged as a therapeutically actionable vulnerability in AML, where specific metabolic adaptations arising as a result of driver mutations or in response to therapy have been reported. Mannose phosphoisomerase (MPI) is the first enzyme in the mannose metabolism (MM) pathway, that leads to the production of GDP-Mannose, a key sugar donor for N-glycosylation reactions. MPI was amongst the top drop-out genes in our published CRISPR-Cas9 screen aiming to identify sensitizers to FLT3 tyrosine kinase inhibitors (TKI) in AML carrying activating FLT3 internal tandem duplication (ITD) mutations (Gallipoli et al., Blood 2018). Moreover, analysis of published genomic datasets indicates that MPI expression levels are higher in AML compared to normal samples, correlate with patient outcome (A) and further increase in paired relapsed to diagnosis samples. We therefore hypothesized that MM and MPI inhibition sensitize AML cells to both FLT3-TKI and standard chemotherapy and tested this in preclinical models.MethodsExperiments were performed in human AML cell lines, primary AML mononuclear cells and normal CD34 + stem/progenitor cells. Liquid chromatography coupled to mass spectrometry and oxygen consumption/extracellular acidification rate as measured by a Seahorse analyser were employed to assess metabolic changes. Gene-expression was measured by RNA-sequencing and confirmed by RT-qPCR. Viability, surface protein expression and lipid peroxidation were assessed by flow-cytometry. Protein expression and localisation was measured by western blot and immunofluorescence. Gene silencing was performed using CRISPR-Cas9 gene editing and inducible short hairpin RNA interference.ResultsWe show that genetic and chemical MPI inhibition sensitizes FLT3 WT and FLT3 ITD AML cell lines in vitro to standard cytarabine chemotherapy and FLT3-TKI respectively and these effects are rescued by the addition of MPI downstream product mannose. We validate these findings in vivo using MPI knock-out (KO) cell line xenografts and in vitro using primary AML samples following MPI knock-down (B). We also demonstrate a lack of toxicity to normal CD34 + cells.Surprisingly, global metabolomic analysis show that MPI KO cells accumulate fatty acids and particularly polyunsaturated fatty acids (PUFA) (C), due to both increased uptake and reduced fatty acid oxidation (FAO). MPI KO cells downregulate several FAO metabolism genes and this is corroborated by the strong positive correlation of MPI expression levels with multiple genes involved in FAO across multiple primary AML datasets. Metabolic profiling demonstrates that MPI KO cells have reduced oxidative phosphorylation metabolism and in particular are unable to oxidise palmitate, an effect rescued by mannose or the FAO activator fenofibrate (D).We link this metabolic defect to the specific activation of the ATF6 arm of the unfolded protein response (UPR) due to a reduction in protein glycosylation in MPI KO cells. We validate this by demonstrating that activating or inhibiting ATF6 respectively phenocopies or rescues the effects of MPI KO (E) and show this to be secondary to ATF6 driving FAO inhibition, via transcriptional downregulation of PPARα, a master regulator of lipid catabolism.Finally, we show that accumulation of PUFA in MPI KO cells is accompanied by reduced cysteine levels and increased markers of oxidative stress and lipid peroxidation, such as 4-hydroxynonenal (4-HNE), all features associated with ferroptosis (F). We validate this by showing that combining FLT3-TKI with MPI KO leads to ferroptotic cell death in AML cells, which can be rescued by the radical-trapping antioxidant ferrostatin.ConclusionsWe provide further evidence to support the role of metabolic rewiring in driving therapy resistance in AML and show for the first time that targeting MPI and MM sensitizes AML cells to cytarabine and FLT3-TKI. Mechanistically we unveil a novel connection between MM and fatty acid metabolism, via activation of the ATF6 arm of the UPR, leading to cellular PUFA accumulation, lipid peroxidation and ferroptotic cell death. Finally, our findings also suggest that triggering ferroptosis can be leveraged as a therapeutically actionable mechanism driving cell death in therapy-resistant AML cells. [Display omitted] DisclosuresVassiliou: Kymab Ltd: Divested equity in a private or publicly-traded company in the past 24 months; STRM.BIO: Consultancy; Astrazeneca: Consultancy.

Preclinical Activity of Selective SYK Inhibitors, Entospletinib and Lanraplenib, Alone or Combined with Targeted Agents in Ex Vivo AML Models with Diverse Mutational Backgrounds

Spleen tyrosine kinase (SYK) is a non-receptor tyrosine kinase that mediates integrin and Fc receptor signaling in myeloid cells. SYK has been implicated as an oncogenic driver in acute myeloid leukemia (AML) with aberrant expression of HOXA9 and MEIS1 and cooperates with FLT3 internal tandem duplication to drive leukemogenesis. The oral SYK inhibitor entospletinib (ENTO) has demonstrated clinical activity in HOXA9/MEIS1 driven AML and is currently being investigated in a phase 3 trial of previously untreated patients with nucleophosmin1-mutated (NPM1 mut) AML. Lanraplenib (LANRA) is a next generation oral SYK inhibitor with potency and selectivity comparable to ENTO. In healthy volunteers and patients with autoimmune disease, LANRA has shown pharmacokinetic properties that compare favorably with ENTO. To support the clinical development of LANRA for the treatment of AML, ex vivo treatment of patient-derived AML cells was used to compare its activity to that of ENTO, both as a single-agent and in combination with other AML therapies. First, ENTO and LANRA single-agent activities were evaluated in peripheral blood-derived blasts from 15 AML patients, representing different mutational backgrounds including NPM1, FLT3, PTPN11, and NRAS mutations. AML cells were seeded into 96 well plates and treated with ENTO and LANRA for 6 days. Comparable effects on viability were observed across the 15 models with the 2 compounds, and in 11 of the models, the half maximal inhibitory concentration (IC 50) values were within 2-fold of each other. ENTO had a slightly lower IC 50 value than LANRA in the FLT3-mutated models possibly due to the direct FLT3 inhibitory activity of ENTO. Next, we tested the activity of ENTO and LANRA ex vivo in bone marrow-derived AML blasts from 29 AML patients representing diverse mutational backgrounds, including NPM1, IDH1, FLT3, and RAS mutations as well as MLL rearrangements. The models were treated for 9 days with either ENTO or LANRA, and viability was assessed using Annexin V and 7-aminoactinomycin D staining. Again, ENTO and LANRA showed comparable effects on cell viability with no significant differences between the compounds when compared across the different mutational backgrounds. Both studies suggest the potential for anti-leukemic activity in several different genetically defined subsets of AML. Matrix combination assays were performed by combining ENTO or LANRA with either cytarabine (NPM1 mut), gilteritinib (FLT3 mut), or trametinib (RAS mut) with cell viability and death assessed after a 3-day incubation period. Increased cell death in an additive manner was observed in all combinations tested, with results for ENTO and LANRA being similar, indicating the utility of both compounds in combinatorial treatment paradigms. These results support the clinical evaluation of LANRA in genetically defined subsets of AML. A phase 1b/2 study of LANRA in combination with the selective FLT3 inhibitor gilteritinib, in patients with relapsed or refractory FLT3 mut AML is planned for the end of this year. DisclosuresDay: Cyteir Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months; Kronos Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Heckman: Novartis: Research Funding; Orion Pharma: Research Funding; Celgene/BMS: Research Funding; Oncopeptides: Consultancy, Research Funding; Kronos Bio, Inc.: Research Funding. Schinzel: Kronos Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Obholzer: Kronos Bio, Inc.: Current Employment, Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company, Current holder of stock options in a privately-held company. Lin: Kronos Bio, Inc.: Current Employment. Kumar: Kronos Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. DiMartino: Kronos Bio, Inc.: Current Employment, Current equity holder in publicly-traded company. Saffran: Kronos Bio, Inc.: Current Employment, Current equity holder in publicly-traded company.

Venetoclax in Combination with Gilteritinib Demonstrates Molecular Clearance of FLT3 mutation in Relapsed/Refractory FLT3-Mutated Acute Myeloid Leukemia

Venetoclax in Combination with Gilteritinib Demonstrates Molecular Clearance of FLT3 mutation in Relapsed/Refractory FLT3-Mutated Acute Myeloid Leukemia

Incidence and Characteristics of Central Nervous System Involvement in Adult Acute Myeloid Leukemia Patients

Features of central nervous system (CNS) involvement in acute myeloid leukemia (AML) are not well-defined. Unlike acute lymphoblastic leukemia, there are limited guidelines for the use of intrathecal chemotherapy with induction for AML, and institutional practices vary. Unfortunately, AML patients with CNS involvement often have worse outcomes compared to those without CNS involvement (Del Principe et al., 2018; Rozovski et al., 2015). It is important to better understand the incidence and predictive clinical and features of CNS involvement in AML to identify patients for diagnostic testing and intrathecal chemotherapy. This retrospective analysis evaluated the incidence and characteristics of CNS involvement in adult AML patients at our institution.CNS screening is recommended for AML patients with specific characteristics, including CNS symptoms, monocytic phenotype, FLT3-internal tandem duplication gene mutation, and hyperleukocytosis (>100,000 white blood cells per microliter). Treatment recommendations for CNS involvement include intrathecal methotrexate, intrathecal cytarabine, or a combination of both.In this IRB-approved retrospective study, medical records were analyzed from the University of Kansas Medical Center from years 2015-2020. Patients were selected by searching the electronic medical record for adult AML patients who received a diagnostic lumbar puncture. Characteristics of interest included presence of CNS symptoms, monocytic phenotype, FLT3 mutation, and hyperleukocytosis. Characteristics that prompted CNS screening were recorded for each patient. Additional data collection included next-generation sequencing (NGS), cytogenetic results, and whether CNS screening occurred during an initial diagnosis or during an AML relapse. Lastly, the number of intrathecal chemotherapy doses was recorded for each patient.The incidence of CNS involvement was determined by dividing the number of newly diagnosed AML CNS-positive patients by the total number of newly diagnosed AML patients. Screening detection rates were determined by dividing the number of CNS-positive patients with each characteristic by the total number that received a lumbar puncture. The percentage of CNS-positive patients with each characteristic was determined by dividing the number with each specific characteristic by the total number of CNS-positive patients. The number of intrathecal chemotherapy doses required for CNS clearance was determined using data only from patients that achieved CNS clearance.Forty-eight patients met current guidelines for CNS screening; 25 were CNS-positive and 23 were CNS-negative (overall screening detection rate of 52%). The incidence of CNS involvement among all newly diagnosed adult AML patients was 3.24%. Screening detection rates were 58% for patients with 3 characteristics, 60% for 2 characteristics, 45% for CNS symptoms, and 30% for monocytic phenotype. Not enough patients presented with the FLT3 mutation or hyperleukocytosis in isolation to calculate accurate detection rates. The CNS-positive group consisted of 28% with 3 characteristics, 24% with 2 characteristics, 20% with CNS symptoms, 12% with monocytic phenotype, 4% with FLT3 mutation, 4% with hyperleukocytosis, and 36% were screened during a relapse rather than at initial diagnosis. Two patients were CNS-positive without any of our characteristics of interest (screened due to extramedullary involvement at presentation and the other was screened as part of pre-transplant evaluation). Seventeen patients achieved CNS clearance with 41% requiring 1 dose of intrathecal chemotherapy, 35% requiring 2 doses, and 24% requiring more than 2 doses. NGS and cytogenetic results did not reveal any additional associations.This data indicates that current AML CNS screening guidelines appear to properly identify a patient population at risk for CNS involvement. At our institution, 52% of screened patients were CNS-positive. This analysis also revealed a diverse distribution of characteristics among AML patients with CNS involvement, and patients commonly presented with multiple risk factors. These results emphasize the need for ongoing research to further delineate CNS involvement risk factors in AML. This data will be used for current policy analysis at our institution and potentially prospective studies to better evaluate outcomes for AML patients with CNS involvement. [Display omitted] DisclosuresLin: AbbVie, Aptevo Therapeutics, Astellas Pharma, Bio-Path Holdings, Celgene, Celyad, Genentech-Roche, Gilead Sciences, Incyte, Jazz Pharmaceuticals, Novartis, Ono Pharmaceutical, Pfizer, Prescient Therapeutics, Seattle Genetics, Tolero, Trovagene: Research Funding.

Phase 1/2, Open-Label, Dose Escalation, Dose Expansion Study of Menin Inhibitor DSP-5336 in Adult Patients (pts) with Acute Leukemia with and without Mixed-Lineage Leukemia (MLL)-Rearrangement or Nucleophosmin 1 (NPM1) Mutation

Relapsed or refractory acute leukemia (AL) in adults, including acute myelogenous leukemia (AML) and acute lymphocytic leukemia (ALL), still represents a significant unmet need. Once AML or ALL relapses after initial treatment, the prognosis is extremely limited, with median survival <6 mo for AML and <1 y for ALL. This is especially true of AML or ALL with MLL rearrangements (MLLr) or relapsed AML with NPM1 mutations. Translocations of the MLL gene, found in approximately 5% to 10% of adult patients with AL, result in an aggressive subtype of leukemia, with a median overall survival (OS) of approximately 9 months and a very high rate of relapse after standard induction therapy for AML or ALL. The NPM1 mutation (NPM1m) is seen in approximately 30-35% of adult AML cases, most commonly in de novo AML, and is associated with a more favorable prognosis in newly diagnosed AML. However, this favorable prognostic effect is lost in the presence of concomitant FLT3-internal tandem duplication (ITD) mutations, which are seen in 40% to 50% of AML with mutated NPM1.These genomic alterations have been the focus for the development of DSP-5336, a novel small-molecule oral therapy that prevents binding of the menin protein to MLL fusion proteins that drive leukemogenesis.In nonclinical studies, DSP-5336 showed selective cell growth inhibitory activity against humanAL cell lines with MLLr or NPM1m. In addition, evidence of antitumor activity and survival advantage was demonstrated in 3 AML xenograft models with MLLr and/or NPM1m. Treatment with DSP-5336 may result in the reduction of leukemic cells, offering therapeutic benefits that include achieving complete remission and prolonged survival in patients with relapsed or refractory AML with MLLr or NPM1m or pts with relapsed or refractory ALL with MLLr.An open-label, single-arm, phase 1/2 study (NCT04988555) of DSP-5336 is being started to evaluate the safety and efficacy of DSP-5336 and determine the recommended phase 2 dose (RP2D) in adults aged ≥18 y with relapsed/refractory AML or ALL after at least one line of standard therapy, including hematopoietic stem cell transplant. Key inclusion criteria are ECOG performance status 0-2 and adequate cardiac, renal, and hepatic function. Patients on the strong CYP3A4 inhibitors ketoconazole, itraconazole, or isavuconazole, are excluded unless they can be safely taken off these medications or switched to another antifungal medication at least 7 days prior to start of study treatment.Patients will be dosed twice daily for 28 days per cycle. The study design is depicted below in Figure 1. In phase 1, 21-30 pts will be enrolled into multiple ascending dose cohorts (1-6 pts per dose level), with dose escalation determined using a two-parameter Bayesian logistic regression model. Determination of the RP2D will be informed by the maximum biologic effect or maximum tolerated dose, whichever is lower. The phase 2 study will include arm A (relapsed or refractory AML with MLLr) and arm B (relapsed or refractory AML with NPM1m) (Figure 1). Patients will be treated at the RP2D to evaluate clinical activity and safety, which will be monitored using Bayesian posterior probability to optimize enrollment with Bayesian stopping rules. Bayesian monitoring of responses will be started after the first 10 enrolled patients are evaluable for efficacy. Study endpoints are summarized in Table 1. [Display omitted] DisclosuresDaver: Genentech: Consultancy, Research Funding; Gilead Sciences, Inc.: Consultancy, Research Funding; Sevier: Consultancy, Research Funding; Hanmi: Research Funding; Trillium: Consultancy, Research Funding; Trovagene: Consultancy, Research Funding; Novartis: Consultancy; ImmunoGen: Consultancy, Research Funding; FATE Therapeutics: Research Funding; Glycomimetics: Research Funding; Abbvie: Consultancy, Research Funding; Astellas: Consultancy, Research Funding; Pfizer: Consultancy, Research Funding; Bristol Myers Squibb: Consultancy, Research Funding; Daiichi Sankyo: Consultancy, Research Funding; Amgen: Consultancy, Research Funding; Novimmune: Research Funding; Jazz Pharmaceuticals: Consultancy, Other: Data Monitoring Committee member; Dava Oncology (Arog): Consultancy; Celgene: Consultancy; Syndax: Consultancy; Shattuck Labs: Consultancy; Agios: Consultancy; Kite Pharmaceuticals: Consultancy; SOBI: Consultancy; STAR Therapeutics: Consultancy; Karyopharm: Research Funding; Newave: Research Funding. Affinito: Sumitomo Dainippon Pharma Oncology, Inc: Current Employment. Cai: Sumitomo Dainippon Pharma Oncology, Inc: Current Employment. Dobrowolska: Sumitomo Dainippon Pharma Oncology, Inc: Current Employment. Dow: Sumitomo Dainippon Pharma Oncology, Inc: Current Employment. Eguchi: Sumitomo Dainippon Pharma: Current Employment. Song: Sumitomo Dainippon Pharma Oncology, Inc: Current Employment. Stoudemire: Sumitomo Dainippon Pharma Oncology, Inc: Current Employment. Watanbe: Sumitomo Dainippon Pharma Oncology, Inc: Current Employment. Komarnitsky: Sumitomo Dainippon Pharma Oncology, Inc: Current Employment.

Frequency of FLT3 Internal Tandem Duplications in Adult Syrian Patients with Acute Myeloid Leukemia and Normal Karyotype.

Objective:Activating mutations of the fms-like tyrosine kinase 3 gene (FLT3) by internal tandem duplications (ITDs) in the juxtamembrane domain (JMD) have been reported in ~30% of adult acute myeloid leukemia (AML) patients with cytogenetically normal karyotype (CN). However, FLT3/ITD mutations are frequently accompanied with leukocytosis, high percentage of blasts in bone marrow (BM), and increased the risk of treatment failure in AML patients. FLT3-ITD mutated AML patients mainly with normal karyotype have higher relapse probability and shorter duration of complete remission (CR) after chemotherapy, so FLT3-ITD mutation is considered as an independent poor prognostic factor in AML. Methods: FLT3-ITD and FLT3-KTD were studied by polymerase chain reaction (PCR) and restriction fragment length polymorphism- PCR (RFLP-PCR) in 44 adults AML patients with cytogenetically normal karyotype (AML-CN) at diagnosis to characterize FLT3 status. The results were correlated with the prognostic factors. Results:In this study, FLT3-ITD mutations were identified in 7 (15.9%) of the 44 AML-CN patients. Among the 7 patients with FLT3/ITD mutations, 6 patients revealed a typical ITDs mutation (fragment size was 329 bp) and one patient showed untypical ITD mutation (fragment size was ~400 bp). Whereas 37 patients (61.7%) were FLT3-ITD. None of all AML-CN patients examined showed FLT3-KTD mutations. Conclusions:Our results support that FLT3-ITD are independent adverse prognostic factors for elderly AML-CN patients and are associated with low overall survival (OS), low rate of CR, high relapse rate (RR), and high percentage of BM blast at diagnosis. We concluded, FLT3 mutation analysis should be performed as a routine test in AML-CN patients.

Clinical impact of NPM1-mutant molecular persistence after chemotherapy for acute myeloid leukemia

AbstractMonitoring of NPM1 mutant (NPM1mut) measurable residual disease (MRD) in acute myeloid leukemia (AML) has an established role in patients who are treated with intensive chemotherapy. The European LeukemiaNet has defined molecular persistence at low copy number (MP-LCN) as an MRD transcript level <1% to 2% with a <1-log change between any 2 positive samples collected after the end of treatment (EOT). Because the clinical impact of MP-LCN is unknown, we sought to characterize outcomes in patients with persistent NPM1mut MRD after EOT and identify factors associated with disease progression. Consecutive patients with newly diagnosed NPM1mut AML who received ≥2 cycles of intensive chemotherapy were included if bone marrow was NPM1mut MRD positive at the EOT, and they were not transplanted in first complete remission. One hundred patients were followed for a median of 23.5 months; 42% remained free of progression at 1 year, either spontaneously achieving complete molecular remission (CRMRD−; 30%) or retaining a low-level NPM1mut transcript (12% for ≥12 months and 9% at last follow-up). Forty percent met the criteria for MP-LCN. Preemptive salvage therapy significantly prolonged relapse-free survival. Risk factors associated with disease progression were concurrent FLT3-internal tandem duplication at diagnosis and suboptimal MRD response (NPM1mut reduction <4.4-log) at EOT.

Gilteritinib Inhibits Glutamine Uptake and Utilization in FLT3-ITD-Positive AML.

Acute myeloid leukemia (AML) with an FLT3 internal tandem duplication (FLT3-ITD) mutation is an aggressive hematologic malignancy associated with frequent relapse and poor overall survival. The tyrosine kinase inhibitor gilteritinib is approved for the treatment of relapse/refractory AML with FLT3 mutations, yet its mechanism of action is not completely understood. Here, we sought to identify additional therapeutic targets that can be exploited to enhance gilteritinib's antileukemic effect. Based on unbiased transcriptomic analyses, we identified the glutamine transporter SNAT1 (SLC38A1) as a novel target of gilteritinib that leads to impaired glutamine uptake and utilization within leukemic cells. Using metabolomics and metabolic flux analyses, we found that gilteritinib decreased glutamine metabolism through the TCA cycle and cellular levels of the oncometabolite 2-hydroxyglutarate. In addition, gilteritinib treatment was associated with decreased ATP production and glutathione synthesis and increased reactive oxygen species, resulting in cellular senescence. Finally, we found that the glutaminase inhibitor CB-839 enhanced antileukemic effect of gilteritinib in ex vivo studies using human primary FLT3-ITD-positive AML cells harboring mutations in the enzyme isocitrate dehydrogenase, which catalyzes the oxidative decarboxylation of isocitrate, producing α-ketoglutarate. Collectively, this work has identified a previously unrecognized, gilteritinib-sensitive metabolic pathway downstream of SLC38A1 that causes decreased glutaminolysis and disruption of redox homeostasis. These findings provide a rationale for the development and therapeutic exploration of targeted combinatorial treatment strategies for this subset of relapse/refractory AML.

Pim kinase inhibitor co-treatment decreases alternative non-homologous end-joining DNA repair and genomic instability induced by topoisomerase 2 inhibitors in cells with FLT3 internal tandem duplication.

Acute myeloid leukemia (AML) with fms-like tyrosine kinase 3 internal tandem duplication (FLT3-ITD) relapses with new chromosome abnormalities following chemotherapy, implicating genomic instability. Error-prone alternative non-homologous end-joining (Alt-NHEJ) DNA double-strand break (DSB) repair is upregulated in FLT3-ITD-expresssing cells, driven by c-Myc. The serine/threonine kinase Pim-1 is upregulated downstream of FLT3-ITD, and inhibiting Pim increases topoisomerase 2 (TOP2) inhibitor chemotherapy drug induction of DNA DSBs and apoptosis. We hypothesized that Pim inhibition increases DNA DSBs by downregulating Alt-NHEJ, also decreasing genomic instability. Alt-NHEJ activity, measured with a green fluorescent reporter construct, increased in FLT3-ITD-transfected Ba/F3-ITD cells treated with TOP2 inhibitors, and this increase was abrogated by Pim kinase inhibitor AZD1208 co-treatment. TOP2 inhibitor and AZD1208 co-treatment downregulated cellular and nuclear expression of c-Myc and Alt-NHEJ repair pathway proteins DNA polymerase θ, DNA ligase 3 and XRCC1 in FLT3-ITD cell lines and AML patient blasts. ALT-NHEJ protein downregulation was preceded by c-Myc downregulation, inhibited by c-Myc overexpression and induced by c-Myc knockdown or inhibition. TOP2 inhibitor treatment increased chromosome breaks in metaphase spreads in FLT3-ITD-expressing cells, and AZD1208 co-treatment abrogated these increases. Thus Pim kinase inhibitor co-treatment both enhances TOP2 inhibitor cytotoxicity and decreases TOP2 inhibitor-induced genomic instability in cells with FLT3-ITD.

Frequency of FLT3 – internal tandem duplication (ITD) gene mutations in various cytogenetic groups in acute myelogenous leukemia (AML): A report from Tamil Nadu, India

ObjectiveTo standardize simple PCR based technique to identify FLT3 mutations in patients with newly diagnosed acute myelogenous leukemia (AML). Material and methodsThirty six patients with different chromosomal presentation were included in the study. DNA was isolated and a regular PCR targeting FLT3-ITD gene mutation was PCR amplified, and observed under agarose gel electrophoresis. Fishers-exact test was performed to evaluate the significance in the mutation presentation in different cytogenetic group and in the male, female ratio. ResultsFLT3-ITD mutation rate in the study population was 10.9%. All patients exhibited a heterozygous mutation; no homozygous mutation was noted in the study. In the cytogenetic group, 3/22 normal karyotype and 3/13 abnormal karyotype presented with the FLT3-ITD mutation. Fishers-exact test did not show any significant difference in various cytogenetic group and the male; female ratio. ConclusionThe heterozygous mutation FLT3 ITD rate in the paper is 10.9%; it is first of its kind of report from Tamil Nadu. No difference was noted in the presentation of the mutation in various cytogenetic groups and in the male, female ratio. PCR based screening technique was established.

Accurate Detection and Quantification of FLT3 Internal Tandem Duplications in Clinical Hybrid Capture Next-Generation Sequencing Data

FLT3 internal tandem duplications (ITDs) are found in approximately one-third of patients with acute myeloid leukemia and have important prognostic and therapeutic implications that have supported their assessment in routine clinical practice. Conventional methods for assessing FLT3-ITD status and allele burden have been primarily limited to PCR fragment size analysis because of the inherent difficulty in detecting large ITD variants by next-generation sequencing (NGS). In this study, we assess the performance of publicly available bioinformatic tools for the detection and quantification of FLT3-ITDs in clinical hybridization-capture NGS data. We found that FLT3_ITD_ext had the highest overall accuracy for detecting FLT3-ITDs and was able to accurately quantify allele burden. Although all other tools evaluated were able to detect FLT3-ITDs reasonably well, allele burden was consistently underestimated. We were able to significantly improve quantification of FLT3-ITD allelic burden independent of the detection method by utilizing soft-clipped reads and/or ITD junctional sequences. In addition, we show that identifying mutant reads by previously identified junctional sequences further improves the sensitivity of detecting FLT3-ITDs in post-treatment samples. Our results demonstrate that FLT3-ITDs can be reliably detected in clinical NGS data using available bioinformatic tools. We further describe how accurate quantification of FLT3-ITD allele burden can be added on to existing clinical NGS pipelines for routineassessment of FLT3-ITD status in patients with acute myeloid leukemia.

Clinical and prognostic relevance of CXCL12 expression in acute myeloid leukemia.

BackgroundAccumulating studies have been made to understand the association between CXC chemokine ligand-12 (CXCL12)/CXC chemokine receptor 4 (CXCR4) and acute myeloid leukemia (AML). However, large-scale data analysis of potential relationship between CXCL12 and AML remains insufficient.MethodsWe collected abundant CXCL12 expression data and AML samples from several publicly available datasets. The CIBERSORT algorithm was used to quantify immune cell fractions and the online website of STRING was utilized for gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. The statistical analysis and graphical work were mainly performed via the R software.ResultsCXCL12 expression was extremely down-regulated in AML. Clinically, low CXCL12 expression was correlated with higher white blood cells (WBCs) (P < 0.0001), more blasts in bone marrow (BM) (P < 0.001) and peripheral blood (PB) (P < 0.0001), FLT3-internal tandem duplications (FLT3-ITD) (P = 0.010) and NPM1 mutations (P = 0.015). More importantly, reduced CXCL12 expression predicted worse overall survival (OS) and event-free survival (EFS) in all AML, non-M3-AML, and cytogenetically normal (CN)-AML patients in three independent cohorts. As for immune cell infiltration, high CXCL12 expressed groups tended to harbor more memory B cells and plasma cells infiltration while low CXCL12 expressed groups exhibited more eosinophils infiltration. GO enrichment and KEGG pathways analysis revealed the potential biological progress the gene participating in.ConclusionsCXCL12 is significantly down-regulated in AML and low CXCL12 expression is an independent and poor predictor of AML prognosis. CXCL12 expression level correlates with clinical and immune characteristics of AML, which could provide potential assistance for treatment. Prospective studies are needed to further validate the impact of CXCL12 expression before routine clinical application in AML.

Abstract 1096: FMS-like tyrosine kinase 3 (FLT3) inhibition by olverembatinib (HQP1351) downregulates MCL-1 and synergizes with BCL-2 inhibitor APG-2575 in preclinical models of FLT3-mutant acute myeloid leukemia (AML)

Abstract AML accounts for 80% of acute leukemias in adults. FLT3 gene mutations are observed in approximately 30% of patients with AML and augur a poor prognosis. Despite antitumor effects of selective FLT3 inhibitors, resistance to these agents continues to pose a formidable clinical challenge. The expression of pro-survival protein BCL-2 is frequently dysregulated, conferring resistance to FLT3 inhibitors, in AML. In this study, we explored the combination of clinical stage multikinase inhibitor HQP1351 (which also targets FLT3) and BCL-2-selective inhibitor APG-2575 in preclinical models of FLT3-mutant AML. Antileukemic activity of HQP1351 was first examined using AML cell lines MV-4-11 and MOLM-13, which harbor FLT3-internal tandem duplication (FLT3/ITD) mutations. In both cell lines, HQP1351 dose-dependently induced cellular apoptosis, APG-2575 also induced apoptosis, and their combination synergistically induced this process. In particular, treatment of subcutaneous MV-4-11 xenograft tumors in nude mice with HQP1351 10 mg/kg significantly suppressed tumor growth, with a tumor growth inhibition (TGI) of 72.4%. This antitumor effect was potentiated when APG-2575 was combined with HQP1351, resulting in a TGI of 97.2%. The combination also improved survival of tumor-bearing mice with a systemic MOLM-13 model compared to either agent alone. Mechanistically, HQP1351 likely inhibited phosphorylation of FLT3 and its downstream signaling pathways, including phosphorylation of AKT, ERK1/2 and STAT5. Taken together, these findings indicate that the antileukemic activity of olverembatinib can be ascribed to on-target inhibition. Single-agent APG-2575 agent triggered cellular apoptosis, as evidenced by cleavage of caspase-3 cleavage and poly(ADP-ribose) polymerase-1 (PARP-1) activation, which are considered hallmarks of apoptosis. Consequently, APG-2575 treatment also increased expression of another antiapoptotic protein (MCL-1), upregulation of which is a pivotal mechanisms of resistance to BCL-2 inhibitors. Notably, HQP1351 substantially decreased cellular MCL-1 expression when combined with APG-2575, further enhancing apoptosis.Taken together, our data suggest that FLT3 inhibition by HQP1351 downregulates MCL-1 and synergizes with BCL-2 inhibitor APG-2575 to potentiate cellular apoptosis in FLT3-mutant AML. The results provide scientific rationale for clinical development of HQP1351 combined with APG-2575 in patients with FLT3-ITD-mutant AML. Citation Format: Douglas D. Fang, Hengrui Zhu, Qiuqiong Tang, Qixin Wang, Na Li, Xu Fang, Ping Min, Guangfeng Wang, Dajun Yang, Yifan Zhai. FMS-like tyrosine kinase 3 (FLT3) inhibition by olverembatinib (HQP1351) downregulates MCL-1 and synergizes with BCL-2 inhibitor APG-2575 in preclinical models of FLT3-mutant acute myeloid leukemia (AML) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1096.

Abstract 3039: Pediatric preclinical testing consortium evaluation of the dual SYK/FLT3 inhibitor TAK-659 in xenograft models of pediatric acute lymphoblastic leukemia

Abstract Introduction: While children with acute lymphoblastic leukemia (ALL) experience close to a 90% likelihood of cure, the outcome for certain high-risk pediatric ALL subtypes remains poor. Spleen Tyrosine Kinase (SYK), a cytosolic nonreceptor tyrosine kinase, is primarily expressed in the hematopoietic lineage and is essential for B-cell receptor signaling. It is associated with malignant transformation, cancer cell proliferation, and is a prominent tyrosine-phosphorylated protein in B-lineage pediatric ALL (Dolai et al, Cancer Res 76:2766-77, 2016). Fms Related Receptor Tyrosine Kinase 3 (FLT3) is a Class III receptor tyrosine kinase that regulates hematopoiesis. While uncommon in ALL, activating FLT3 mutations and internal tandem duplications are associated with poor outcome. TAK-659 is a dual SYK/FLT3 reversible inhibitor currently undergoing clinical trials against B-cell lymphoma, acute myeloid leukemia and solid tumors. Therefore, it was of interest for the Pediatric Preclinical Testing Consortium to test TAK-659 in vivo against its patient-derived xenograft (PDX) models of pediatric ALL. Methods: SYK and FLT3 mRNA expression in ALL PDXs was quantified by RNAseq (https://pedcbioportal.org) and 8 B-lineage pediatric ALL PDXs (3 B-cell precursor, BCP; 4 mixed-lineage leukemia-rearranged, MLLr; one Philadelphia Chromosome-like, Ph-like) were selected for testing. Engraftment was evaluated by enumerating the proportion of human CD45+ cells (%huCD45+) in the peripheral (PB) blood at weekly intervals. TAK-659 was tested at 60 mg/kg by oral gavage daily for 21 days. Events were defined as %huCD45+ ≥ 25%. At Day 28 post-treatment initiation or event (whichever occurred first), 4 mice/group were humanely killed to assess leukemia infiltration in the spleen and bone marrow (BM). Drug efficacy was assessed by event-free survival (EFS) of treated (T) and control (C) groups by T-C, T/C and stringent objective response measures (Houghton et al, Pediatr Blood Cancer 49:928-40, 2007). Results: The in vivo efficacy of TAK-659 was evaluated against B-lineage PDXs as FLT3 and SYK mRNA expression was higher in the PDXs from this lineage compared with PDXs from T-ALL. TAK-659 was well tolerated and significantly prolonged the time to event in 6/8 PDXs (T-C 0-25.5 days, T/C 1.0-2.2). Only one MLLr-ALL obtained an objective response (partial response) with all other models exhibiting progressive disease. The minimum mean %huCD45+ was significantly reduced in 5/8 PDXs in TAK-659 treated mice compared to control. Despite significant reductions in %huCD45+ in the PB for 5/8 PDXs, TAK-659 did not significantly reduce the spleen or BM infiltration of any PDXs. Conclusion: TAK-659 exhibited low to moderate single-agent in vivo activity against pediatric B-ALL PDXs representative of diverse disease subtypes. (Supported by NCI Grants CA199000 and CA199922) Citation Format: Richard B. Lock, Kathryn Evans, Narimanne El-Zein, Eric J. Earley, Stephen W. Erickson, Beverly A. Teicher, Malcolm A. Smith. Pediatric preclinical testing consortium evaluation of the dual SYK/FLT3 inhibitor TAK-659 in xenograft models of pediatric acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3039.

Abstract 1257: HM43239, a novel FLT3 inhibitor, has the potential to inhibit mutations resistant to FLT3 inhibitors

Abstract Acute myeloid leukemia (AML) with FMS-like tyrosine kinase 3 (FLT3) mutations is associated with poor prognosis with a high risk of relapse after therapy and reduces overall survival. Approximately 30 % of AML patients carry FLT3 internal tandem duplication (ITD) or tyrosine kinase domain (TKD) mutations. Sustained FLT3 inhibition can result in the emergence of resistance-conferring genetic alteration in TKD domain, usually at residues D835 and F691. Therefore, acquired TKD mutations has become a critical therapeutic target in AML therapy. On the other hand, SYK is one of kinases deeply implicated in many hematologic malignancies and highly activated in FLT3 mutation AML. SYK overexpression is known to promote over transformation of FLT3 driven AML and induce resistance to FLT3 targeted therapy. In this study, we characterized HM43239, a novel FLT3 inhibitor with SYK inhibitory activity, and assessed its potential as a novel therapeutic agent to overcome the resistances against AML therapy using current FLT3 inhibitors. HM43239 is an orally active small molecule inhibitor and it exhibited sub-nanomolar potency on binding affinity for FLT3 wild type, ITD, TKDs and ITD/TKDs mutants. HM43239 potently inhibited phosphorylation of FLT3 and its downstream such as p-STAT3/p-STAT5 dose dependently in both MOLM-14 cells harboring FLT3 ITD/F691L and FLT3 ITD/D835Y. In KG-1a cells, HM43239 potently inhibited phosphorylation of SYK, STAT3 and STAT5. Moreover, it inhibited the proliferation and induced the apoptosis of leukemic stem cell (LSC) marker-expressing KG-1a cells (CD34+/CD38- cells), suggesting the possibility of targeting LSC. Furthermore, HM43239 showed to be exhibited good inhibitory activity against FLT3 mutated AML cell lines and effectively regress the tumors in MOLM-14 cells expressing FLT3 ITD/F691L or FLT3 ITD/D835Y xenograft mice models. On the other hand, it was confirmed that HM43239 strongly inhibited the phosphorylation of SYK in the medium of co-culture of stromal cells and AML cells. As a result, HM43239 alone more effectively induced tumor regression and prolonged the survival duration of animals than an approved FLT3 inhibitor (e.g. gilteritinib) in resistant FLT3 ITD/D835Y or ITD/F691L mutated MOLM-14 xenograft mice models. These results suggest that HM43239 could overcome the resistance induced by bone marrow microenvironment in AML patients. Taken together, HM43239 showed strong anticancer activity through various in vitro and in vivo preclinical models of AML, implicating the mechanism of overcoming resistance and preventing relapse. The effect of HM43239 in human would be demonstrated in ongoing Phase I/II clinical trials (NCT03850574) to develop promising therapeutics for patients with AML. Citation Format: Inhwan Bae, Jaeyul Choi, Jiyoung Song, Joo-Yun Byun, Eunyoung Lee, Taehun Song, Yu-Yon Kim, Yesol Bak, Young Hoon Kim, Young Gil Ahn, Kwee Hyun Suh. HM43239, a novel FLT3 inhibitor, has the potential to inhibit mutations resistant to FLT3 inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1257.