Inhibition of NOTCH4 Sensitizes FLT3/ITD Acute Myeloid Leukemia Cells to FLT3 Tyrosine Kinase Inhibition

Internal tandem duplication mutations of FLT3 (FLT3/ITD) confer poor prognosis in AML. FLT3 tyrosine kinase inhibitors (TKIs) alone have limited and transient clinical efficacy thus calling for new targets for more effective combination therapy. In a loss-of-function RNAi screen, we identified NOTCH4 as one such potential target whose inhibition proved cytotoxic to AML cell lines, and also sensitized them to FLT3 inhibition. Further investigation found increased NOTCH4 expression in FLT3/ITD AML cell lines and primary patient samples. Inhibition of NOTCH4 by shRNA knockdown, CRISPR-Cas9-based knockout or g-secretase inhibitors synergized with FLT3 TKIs to kill FLT3/ITD AML cell lines and patient samples. NOTCH4 inhibition also sensitized TKI-resistant FLT3/ITD cell lines to FLT3 TKI inhibition. The combination reduced phospho-ERK and phospho-AKT, indicating inhibition of MAPK and PI3K/AKT signaling pathways. It also led to changes in the expression of genes involved in regulating cell cycling, DNA repair and transcription. A patient-derived xenograft model showed the combination reduced both the level of leukemic involvement of primary human FLT3/ITD AML cells and their ability to engraft secondary recipients. In summary, these results demonstrate that NOTCH4 inhibition synergizes with FLT3 TKIs to eliminate FLT3/ITD AML cells, providing a new therapeutic target for AML patients with FLT3/ITD mutations.