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.