Targeting FLT3 Mutation in Acute Myeloid Leukemia: Current Strategies and Future Directions

Tracking Response and Resistance in Acute Myeloid Leukemia through Single-Cell DNA Sequencing Helps Uncover New Therapeutic Targets

Acute myeloid leukemia (AML) is an aggressive hematologic neoplasia with a complex polyclonal architecture. Among driver lesions, those involving the FLT3 gene represent the most frequent mutations identified at diagnosis. The development of tyrosine kinase inhibitors (TKIs) has improved the clinical outcomes of FLT3-mutated patients (Pt). However, overcoming resistance to these drugs remains a challenge. To unravel the molecular mechanisms underlying therapy resistance and clonal selection, we conducted a longitudinal analysis using a single-cell DNA sequencing approach (MissionBioTapestri® platform, San Francisco, CA, USA) in two patients with FLT3-mutated AML. To this end, samples were collected at the time of diagnosis, during TKI therapy, and at relapse or complete remission. For Pt #1, disease resistance was associated with clonal expansion of minor clones, and 2nd line TKI therapy with gilteritinib provided a proliferative advantage to the clones carrying NRAS and KIT mutations, thereby responsible for relapse. In Pt #2, clonal architecture was less complex, and 1st line TKI therapy with midostaurin was able to eradicate the leukemic clones. Our results corroborate previous findings about clonal selection driven by TKIs, highlighting the importance of a deeper characterization of individual clonal architectures for choosing the best treatment plan for personalized approaches aimed at optimizing outcomes.

BRD4 degraders may effectively counteract therapeutic resistance of leukemic stem cells in AML and ALL

Acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) are life-threatening hematopoietic malignancies characterized by clonal expansion of leukemic blasts in the bone marrow and peripheral blood. The epigenetic reader BRD4 and its downstream effector MYC have recently been identified as potential drug targets in human AML and ALL. We compared anti-leukemic efficacies of the small-molecule BET inhibitor JQ1 and the recently developed BRD4 degraders dBET1 and dBET6 in AML and ALL cells. JQ1, dBET1, and dBET6 were found to suppress growth and viability in all AML and ALL cell lines examined as well as in primary patient-derived AML and ALL cells, including CD34+/CD38- and CD34+/CD38+ leukemic stem and progenitor cells, independent of the type (variant) of leukemia or molecular driver expressed in leukemic cells. Moreover, we found that dBET6 overcomes osteoblast-induced drug resistance in AML and ALL cells, regardless of the type of leukemia or the drug applied. Most promising cooperative or even synergistic drug combination effects were seen with dBET6 and the FLT3 ITD blocker gilteritinib in FLT3 ITD-mutated AML cells, and with dBET6 and the multi-kinase blocker ponatinib in BCR::ABL1+ ALL cells. Finally, all BRD4-targeting drugs suppressed interferon-gamma- and tumor necrosis factor-alpha-induced expression of the resistance-related checkpoint antigen PD-L1 in AML and ALL cells, including LSC. In all assays examined, the BRD4 degrader dBET6 was a superior anti-leukemic drug compared with dBET1 and JQ1. Together, BRD4 degraders may provide enhanced inhibition of multiple mechanisms of therapy resistance in AML and ALL.

Advances in the pathogenesis of FLT3 -mutated acute myeloid leukemia and targeted treatments

PURPOSE OF REVIEW

FLT3 mutations are among the most common myeloid drivers identified in adult acute myeloid leukemia (AML). Their identification is crucial for the precise risk assessment because of the strong prognostic significance of the most recurrent type of FLT3 alterations, namely internal tandem duplications (ITDs). Recent advances in the pathogenesis and biology of FLT3 -mutated AML have opened an opportunity for development and application of selective inhibition of FLT3 pathway.

RECENT FINDINGS

In the last decade, at least three targeted treatments have been approved by regulatory agencies and several others are currently under investigations. Here, we review the latest advance in the role of FLT3 mutations in AML, providing an outline of the available therapeutic strategies, their mechanisms of actions and of resistance, as well as routes for potential improvement.

SUMMARY

The availability of FLT3 inhibitors has improved outcomes in AML harboring such mutations, currently also reflected in disease stratification and recommendations. Newer inhibitors are under investigations, and combinations with chemotherapy or other targeted treatments are being explored to further improve disease outcomes.

Co-occurrence of Erdheim-Chester disease and clonally evolving acute myeloid leukemia with FLT3-ITD and PTPN11 mutations

Erdheim-Chester disease (ECD) is a rare histiocytosis that tends to co-exist with other myeloid malignancies. Here, we use genetic and transcriptomic sequencing to delineate a case of co-occurring BRAFV600E-mutated ECD and acute myeloid leukemia (AML), followed by AML remission and relapse. The AML relapse involved the extinction of clones with KMT2A-AFDN and FLT3-ITD, and the predominance of PTPN11-mutated subclones with distinct transcriptomic features. This case report has highlighted the screening for other myeloid malignancies at the diagnosis of ECD and the clinical significance of PTPN11-mutated AML subclones that require meticulous monitoring.

FLT3-Mutated Leukemic Stem Cells: Mechanisms of Resistance and New Therapeutic Targets

Despite the availability of target drugs in the first and second line, only 30% of FLT3mut AMLs are cured. Among the multiple mechanisms of resistance, those of FLT3mut LSC are the most difficult to eradicate because of their metabolic and genomic characteristics. Reactivation of glycogen synthesis, inhibition of the RAS/MAPK pathway, and degradation of FLT3 may be potential aids to fight the resistance of LSC to FLT3i. LSC is also characterized by the expression of a CD34+/CD25+/CD123+/CD99+ immunophenotype. The receptor and ligand of FLT3, the natural killer group 2 member D ligand (NKGD2L), and CD123 are some of the targets of chimeric antigen receptor T cells (CAR-T), bispecific T-cell engager molecules (BiTEs), CAR-NK and nanoparticles recently designed and reported here. The combination of these new therapeutic options, hopefully in a minimal residual disease (MRD)-driven approach, could provide the future answer to the challenge of treating FLT3mut AML.

N-(3-Methoxyphenyl)-6-(7-(1-methyl-1H-pyrazol-4-yl)imidazo[1,2-a]pyridin-3-yl)pyridin-2-amine is an inhibitor of the FLT3-ITD and BCR-ABL pathways, and potently inhibits FLT3-ITD/D835Y and FLT3-ITD/F691L secondary mutants

Activating mutations within FLT3 make up 30 % of all newly diagnosed acute myeloid leukemia (AML) cases, with the most common mutation being an internal tandem duplication (FLT3-ITD) in the juxtamembrane region (25 %). Currently, two generations of FLT3 kinase inhibitors have been developed, with three inhibitors clinically approved. However, treatment of FLT3-ITD mutated AML is limited due to the emergence of secondary clinical resistance, caused by multiple mechanism including on-target FLT3 secondary mutations - FLT3-ITD/D835Y and FLT3-ITD/F691L being the most common, as well as the off-target activation of alternative pathways including the BCR-ABL pathway. Through the screening of imidazo[1,2-a]pyridine derivatives, N-(3-methoxyphenyl)-6-(7-(1-methyl-1H-pyrazol-4-yl)imidazo[1,2-a]pyridin-3-yl)pyridin-2-amine (compound 1) was identified as an inhibitor of both the FLT3-ITD and BCR-ABL pathways. Compound 1 potently inhibits clinically related leukemia cell lines driven by FLT3-ITD, FLT3-ITD/D835Y, FLT3-ITD/F691L, or BCR-ABL. Studies indicate that it mediates proapoptotic effects on cells by inhibiting FLT3 and BCR-ABL pathways, and other possible targets. Compound 1 is more potent against FLT3-ITD than BCR-ABL, and it may have other possible targets; however, compound 1 is first step for further optimization for the development of a balanced FLT3-ITD/BCR-ABL dual inhibitor for the treatment of relapsed FLT3-ITD mutated AML with multiple secondary clinical resistant subtypes such as FLT3-ITD/D835Y, FLT3-ITD/F691L, and cells co-expressing FLT3-ITD and BCR-ABL.

Developments of Fms-like Tyrosine Kinase 3 Inhibitors as Anticancer Agents for AML Treatment

BACKGROUND

FMS-like tyrosine kinase 3 (FLT3) is a commonly mutated gene in acute myeloid leukemia. As a receptor tyrosine kinase (RTK), FLT3 plays a role in the proliferation and differentiation of hematopoietic stem cells. As the most frequent molecular alteration in AML, FLT3 has drawn the attention of many researchers, and a lot of small molecule inhibitors targeting FLT3 have been intensively investigated as potential drugs for AML therapy.

METHODS

In this paper, PubMed and SciFinder® were used as a tool; the publications about "FLT3 inhibitor" and "Acute myeloid leukemia" were surveyed from 2014 to the present with an exclusion of those published as patents.

RESULTS

In this study, the structural characterization and biological activities of representative FLT3 inhibitors were summarized. The major challenges and future directions for further research are discussed.

CONCLUSION

Recently, numerous FLT3 inhibitors have been discovered and employed in FLT3-mutated AML treatment. In order to overcome the drug resistance caused by FLT3 mutations, screening multitargets FLT3 inhibitors has become the main research direction. In addition, the emergence of irreversible FLT3 inhibitors also provides new ideas for discovering new FLT3 inhibitors.

Understanding the Role of Activation Loop Mutants in Drug Efficacy for FLT3-ITD

The type III receptor tyrosine kinase FLT3 is a pivotal kinase for hematopoietic progenitor cell regulation, with significant implications in acute myeloid leukemia (AML) through mutations like internal tandem duplication (ITD). This study delves into the structural intricacies of FLT3, the roles of activation loop mutants, and their interaction with tyrosine kinase inhibitors. Coupled with this, the research leverages molecular contrastive learning and protein language modeling to examine interactions between small molecule inhibitors and FLT3 activation loop mutants. Utilizing the ConPLex platform, over 5.7 million unique FLT3 activation loop mutants-small molecule pairs were analyzed. The binding free energies of three inhibitors were assessed, and cellular apoptotic responses were evaluated under drug treatments. Notably, the introduction of the Xepto50 scoring system provides a nuanced metric for drug efficacy. The findings underscore the modulation of molecular interactions and cellular responses by Y842 mutations in FLT3-KD, highlighting the need for tailored therapeutic approaches in FLT3-ITD-related malignancies.

AML under the Scope: Current Strategies and Treatment Involving FLT3 Inhibitors and Venetoclax-Based Regimens

Acute myeloid leukemia (AML) is a disease that mainly affects elderly patients who are more often unfit for intensive chemotherapy (median age of diagnosis is 68). The regimens, including venetoclax, a highly specific BCL-2 (B-cell lymphoma-2) inhibitor, are a common alternative because of their safer profile and fewer side effects. However, the resistance phenomenon of leukemic cells necessitates the search for drugs that would help to overcome the resistance and improve treatment outcomes. One of the resistance mechanisms takes place through the upregulation of MCL-1 and BCL-XL, preventing BAX/BAK-driven MOMP (mitochondrial outer membrane permeabilization), thus stopping the apoptosis process. Possible partners for BCL-2 inhibitors may include inhibitors from the FLT3i (FMS-like tyrosine kinase-3 inhibitor) group. They resensitize cancer cells through the downregulation of MCL-1 expression in the FLT3 mutated cells, resulting in the stronger efficacy of BCL-2 inhibitors. Also, they provide an additional pathway for targeting the clonal cell. Both preclinical and clinical data suggest that the combination might show a synergistic effect and improve patients' outcomes. The aim of this review is to determine whether the combination of venetoclax and FLT3 inhibitors can impact the therapeutic approaches and what other agents they can be combined with.

A Review of FLT3 Kinase Inhibitors in AML

Acute myeloid leukemia (AML) is a highly aggressive illness distinguished by the accumulation of abnormal hematopoietic precursors in both the bone marrow and peripheral blood. The prevalence of FLT3 gene mutations is high and escalates the probability of relapse and mortality. The survival rates for AML patients, particularly those over 65, are low. FLT3 mutation screening at diagnosis is mandatory, and FLT3 inhibitors are crucial in treating AML patients with mutations. There are two categories of FLT3 mutations: FLT3-ITD located in the juxtamembrane domain and FLT3-TKD in the tyrosine kinase domain. FLT3-ITD is the most common type, affecting nearly a quarter of patients, whereas FLT3-TKD only affects 6-8% of patients. FLT3 inhibitors are now crucial in treating AML patients with FLT3 mutations. When dealing with FLT3-mutated AML, the recommended course of treatment typically involves chemotherapy and midostaurin, followed by allogeneic hematopoietic cell transplantation (HCT) to maximize the likelihood of success. Maintenance therapy can lower the risk of relapse, and gilteritinib is a better option than salvage chemotherapy for relapsed or refractory cases. Clinical trials for new or combined therapies are the most effective approach. This review discusses treatment options for patients with FLT3-mutated AML, including induction chemotherapy and options for relapsed or refractory disease. Additional treatment options may become available as more studies are conducted based on the patient's condition and susceptibility.

A Leukemic Target with a Thousand Faces: The Mitochondria

In the era of personalized medicine greatly improved by molecular diagnosis and tailor-made therapies, the survival rate of acute myeloid leukemia (AML) at 5 years remains unfortunately low. Indeed, the high heterogeneity of AML clones with distinct metabolic and molecular profiles allows them to survive the chemotherapy-induced changes, thus leading to resistance, clonal evolution, and relapse. Moreover, leukemic stem cells (LSCs), the quiescent reservoir of residual disease, can persist for a long time and activate the recurrence of disease, supported by significant metabolic differences compared to AML blasts. All these points highlight the relevance to develop combination therapies, including metabolism inhibitors to improve treatment efficacy. In this review, we summarized the metabolic differences in AML blasts and LSCs, the molecular pathways related to mitochondria and metabolism are druggable and targeted in leukemia therapies, with a distinct interest for Venetoclax, which has revolutionized the therapeutic paradigms of several leukemia subtype, unfit for intensive treatment regimens.