Targeting ribonucleotide reductase to overcome FLT3 inhibitor resistance in acute myeloid leukemia

Acute myeloid leukemia (AML) is a malignant hematopoietic disorder characterized by the clonal proliferation and impaired differentiation of myeloid progenitor cells. It is the most common type of acute leukemia in adults and is associated with high rates of chemotherapy resistance and relapse. Mutations in the FMS-like tyrosine kinase 3 ( FLT3 ) gene occur in approximately 30% of AML cases and are linked to poor prognosis. The discovery of FLT3 inhibitors (FLT3i) has represented a breakthrough in targeted AML therapy; However, resistance to FLT3i remains a significant clinical challenge that limits long-term efficacy. Previously, we identified Sprouty RTK Signaling Antagonist 3 ( SPRY3 )-a negative regulator of RAS/MAPK signaling-as a key determinant of FLT3i sensitivity. Loss of SPRY3 activated RAS signaling and conferred robust FLT3i resistance in AML. Consistent with these findings, activating NRAS mutations (such as NRAS G12C and NRAS Q61K ) -present in approximately 15% of AML cases-also drive resistance to FLT3i. In our recent studies, we identified ribonucleotide reductase (RNR) as a critical downstream effector upregulated by NRAS -activating mutations ( NRAS mut ) in AML, driving FLT3i resistance. RNR catalyzes the conversion of ribonucleoside diphosphates (NDPs) into deoxyribonucleoside diphosphates (dNDPs), which are subsequently phosphorylated to deoxyribonucleoside triphosphates (dNTPs)-the essential building blocks for DNA replication and repair. We found that NRAS mut AML cells exhibit significantly elevated expression of RRM1 , RRM2 , and RRM2B , the three subunits of RNR, following FLT3i treatment. Mechanistically, RAS activation enhances RNR expression through the RAS-MAPK-E2F1/MYC signaling axis. Inhibition of E2F1 or MYC markedly reduced RRM1 and RRM2 expression, confirming their transcriptional regulation of RNR. Furthermore, knockdown of Dual Specificity Phosphatase 6 (DUSP6)-a negative feedback regulator of RAS/MAPK signaling-further increased RNR activity and sensitized NRAS mut AML cells to RNR inhibition. Functionally, both pharmacologic inhibition of RNR using clofarabine and siRNA-mediated knockdown of RNR subunits effectively restored FLT3i sensitivity in NRAS mut AML. Combination therapy with gilteritinib and clofarabine in both cell line-derived xenograft (CDX) and patient-derived xenograft (PDX) models significantly reduced leukemic burden and prolonged survival compared with either agent alone. In summary, our findings identify RNR as a critical downstream effector of RAS/MAPK signaling and a promising therapeutic target to overcome FLT3i resistance in NRAS mut AML. These results provide a strong mechanistic and preclinical rationale for the combined inhibition of FLT3 and RNR as a novel therapeutic strategy to improve outcomes in resistant AML.