Tyrosine Kinase Inhibitor-Induced Defects in DNA Repair Sensitize FLT3(ITD)-Positive Acute Myeloid Leukemia Quiescent and Proliferative Cells to PARP Inhibitors

Mutations in the FMS-like tyrosine-kinase 3 (FLT3) such as internal tandem duplications (ITD) can be found in up to 23% of patients with acute myeloid leukemia (AML) and confer a poor prognosis. Current treatment options for FLT3(ITD)-positive AMLs include genotoxic therapy and FLT3 inhibitors, which are not curative. Poly (ADP-ribose) polymerase (PARP) inhibitors have been recently successfully applied to induce synthetic lethality in tumors harboring BRCA1/2 mutations and displaying deficiency in a major DNA double strand break (DSB) repair pathway - homologous recombination (HR). Using personalized medicine approach we recently reported that a cohort of AMLs displayed deficiencies in HR and also in another major DSB repair pathway - non-homologous end-joining (NHEJ) (Nieborowska-Skorska et al., J.Clin.Invest., 2017). HR and NHEJ play a key roles in DSB repair in proliferating (HR and NHEJ) and quiescent (NHEJ) cells, and HR/NHEJ deficiency sensitized quiescent and proliferating AML stem cells to synthetic lethality triggered by PARP inhibitors olaparib and BMN673 (talazoparib). Although AML cells expressing FLT3(ITD) displayed activation of HR and NHEJ activities, we show here that inhibition of FLT3(ITD) kinase by tyrosine kinase inhibitor AC220 caused downregulation of selected DNA repair proteins such as BRCA1, BRCA2, PALB2, RAD51, and LIG4, resulting in simultaneous inhibition of HR and NHEJ. PARP inhibitors olaparib and BMN673 caused accumulation of lethal DSBs and cell death in AC220-treated FLT3(ITD)-positive leukemia cells thus mimicking synthetic lethality. In addition, combination of FLT3 inhibitor and PARP inhibitor in vitro eliminated FLT3(ITD)-positive quiescent and proliferating leukemia stem cells and leukemic progenitors from human and mouse leukemia samples. Moreover, AC220 combined with BMN673 were highly effective in vivo against FLT3(ITD)-positive primary AML xenograft cells, including leukemia initiating cells. In conclusion, we postulate that FLT3 inhibitor-induced deficiencies in HR and NHEJ sensitized FLT3(ITD)-positive AML cells to synthetic lethality triggered by PARP inhibitor. Therefore FLT3(ITD) could be used as a precision medicine marker identifying patients with AMLs, which may benefit from therapeutic regimen combining FLT3 and PARP inhibitors.