FLT3 Inhibition in Acute Myeloid Leukaemia - Current Knowledge and Future Prospects

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.

Design, synthesis, and evaluation of pyrido.[3,4-b]pyrazin-2(1H)-one derivatives as potent FLT3 inhibitors

Acute myeloid leukemia (AML) is characterized by fast progression and low survival rates, in which Fms-like tyrosine kinase 3 (FLT3) receptor mutations have been identified as driver mutations in a subgroup of AML patients. Herein, we describe the design, synthesis, and biological evaluation of a novel series of potent pyrido.[3,4-b]pyrazin-2(1H)-one derivatives as FLT3 inhibitors. The compounds exhibited moderate to potent FLT3 kinase inhibitory potency and excellent antiproliferative activities against MV4-11 cells. Among them, compound 13 demonstrated the most potent kinase activity against FLT3-D835Y (IC50 = 29.54 ± 4.76 nM) and cellular potency against MV4-11 cells (IC50 = 15.77 ± 0.15 nM). Compound 13 also efficiently inhibited the growth of multiple mutant BaF3 cells expressing FLT3-D835V/F, FLT3-F691L, and FLT3-ITD/D835Y. Furthermore, compound 13 was metabolically stable in mouse liver microsomes. Moreover, the treatment with compound 13 led to robust inhibition of FLT3 autophosphorylation on Tyr589/591 in MV4-11 cells. In summary, our data demonstrated that 13 was worthy of further study for the treatment of AML.

Targeting Gatekeeper Mutations for Kinase Drug Discovery

Clinically acquired resistance is a major challenge in cancer therapies with small-molecule kinase inhibitors (SMKIs). Gatekeeper mutations in the ATP-binding pocket of kinases are the most common mutations leading to acquired resistance. To date, seven new-generation kinase inhibitors targeting gatekeeper mutations have been approved by the FDA; however, the clinical need is still unmet. Here, we systematically summarize the types of gatekeeper mutations across the kinase family, the structural basis for acquired resistance, and newly developed SMKIs targeting gatekeeper mutations as well as highlight the opportunities and challenges of kinase drug discovery for targeting gatekeeper mutations.

Identification of 2-Aminopyrimidine Derivatives as FLT3 Kinase Inhibitors with High Selectivity over c-KIT

Herein, we report two promising compounds 30 and 36 possessing nanomolar FLT3 inhibitory activities (IC₅₀ = 1.5-7.2 nM), high selectivity over c-KIT (>1000-fold), and excellent anti-AML activity (MV4-11 IC₅₀ = 0.8-3.2 nM). Furthermore, these two compounds efficiently inhibited the growth of multiple mutant BaF3 cells expressing FLT3-ITD, FLT3-D835V/F, FLT3-F691L, FLT3-ITD-F691L, and FLT3-ITD-D835Y. Oral administration of 30 and 36 at 6 mg/kg/d could significantly suppress tumor growth in the MV4-11 cell-inoculated xenograft model, exhibiting tumor growth inhibitory rates of 83.5% and 95.1%, respectively. Importantly, 36 could prolong the mouse survival time in the FLT3-ITD-TKD dual mutation syngeneic mouse model (BaF3-FLT3-ITD-D835Y) at a dose of 6 mg/kg p.o. bid/4W. No clear myelosuppression was observed in the treated group of 36 in the MPO strain of zebrafish, even at 10 μM. In summary, our data demonstrated that 36 may represent a promising candidate for the treatment of FLT3 mutant AML.

Sequential Metal Catalysis towards 7-Oxostaurosporine and Its Non-Natural Septanose Analogue

We report sequential metal catalysis towards indolocarbazole glycosides. The signature event is highlighted by i) Pd⁰ -catalyzed addition of indolocarbazole to alkoxyallene combined with ring-closing-metathesis; ii) Ru-catalyzed chemoselective olefin migration; iii) Pd^II -catalyzed oxidative cyclization to build the bicyclic core structure of the target compounds. This approach gave access to both natural pyranose- and non-natural septanose glycosides. A short formal synthesis of 7-oxostaurosporine was achieved via this strategy.

Enhanced cytarabine-induced killing in OGG1-deficient acute myeloid leukemia cells

Human clinical trials suggest that inhibition of enzymes in the DNA base excision repair (BER) pathway, such as PARP1 and APE1, can be useful in anticancer strategies when combined with certain DNA-damaging agents or tumor-specific genetic deficiencies. There is also evidence suggesting that inhibition of the BER enzyme 8-oxoguanine DNA glycosylase-1 (OGG1), which initiates repair of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) and 2,6-diamino-4-hydroxy-5-formamidopyrimidine (Fapy-dG), could be useful in treating certain cancers. Specifically, in acute myeloid leukemia (AML), both the RUNX1-RUNX1T1 fusion and the CBFB-MYH11 subtypes have lower levels of OGG1 expression, which correlate with increased therapeutic-induced cell cytotoxicity and good prognosis for improved, relapse-free survival compared with other AML patients. Here we present data demonstrating that AML cell lines deficient in OGG1 have enhanced sensitivity to cytarabine (cytosine arabinoside [Ara-C]) relative to OGG1-proficient cells. This enhanced cytotoxicity correlated with endogenous oxidatively-induced DNA damage and Ara-C-induced DNA strand breaks, with a large proportion of these breaks occurring at common fragile sites. This lethality was highly specific for Ara-C treatment of AML cells deficient in OGG1, with no other replication stress-inducing agents showing a correlation between cell killing and low OGG1 levels. The mechanism for this preferential toxicity was addressed using in vitro replication assays in which DNA polymerase δ was shown to insert Ara-C opposite 8-oxo-dG, resulting in termination of DNA synthesis. Overall, these data suggest that incorporation of Ara-C opposite unrepaired 8-oxo-dG may be the fundamental mechanism conferring selective toxicity and therapeutic effectiveness in OGG1-deficient AML cells.

Novel Approaches to Target Mutant FLT3 Leukaemia

Simple Summary

Acute myeloid leukemia (AML) is a haematologic disease in which oncogenic mutations in the receptor tyrosine kinase FLT3 frequently lead to leukaemic development. Potent treatment of AML patients is still hampered by inefficient targeting of leukemic stem cells expressing constitutive active FLT3 mutants. This review summarizes the current knowledge about the regulation of FLT3 activity at cellular level and discusses therapeutical options to affect the tumor cells and the microenvironment to impair the haematological aberrations.

Abstract

Fms-like tyrosine kinase 3 (FLT3) is a member of the class III receptor tyrosine kinases (RTK) and is involved in cell survival, proliferation, and differentiation of haematopoietic progenitors of lymphoid and myeloid lineages. Oncogenic mutations in the FLT3 gene resulting in constitutively active FLT3 variants are frequently found in acute myeloid leukaemia (AML) patients and correlate with patient’s poor survival. Targeting FLT3 mutant leukaemic stem cells (LSC) is a key to efficient treatment of patients with relapsed/refractory AML. It is therefore essential to understand how LSC escape current therapies in order to develop novel therapeutic strategies. Here, we summarize the current knowledge on mechanisms of FLT3 activity regulation and its cellular consequences. Furthermore, we discuss how aberrant FLT3 signalling cooperates with other oncogenic lesions and the microenvironment to drive haematopoietic malignancies and how this can be harnessed for therapeutical purposes.