Preclinical Efficacy of MEK Inhibition in Nras Mutant Acute Myeloid Leukemia

Oncogenic NRAS mutations are highly prevalent in hematologic malignancies. In acute myeloid leukemia (AML), genetic analysis supports the hypothesis that NRAS mutations cooperate with antecedent molecular lesions in leukemogenesis. Furthermore, NRAS mutations identified at diagnosis may disappear at relapse, raising questions regarding the potential clinical benefits of inhibiting oncogenic N-Ras in AML. To directly investigate the consequences of Nras inactivation in normal hematopoiesis, we used the Mx1-Cre transgene to inactivate a conditional mutant Nras allele and analyzed hematopoiesis and hematopoietic stem and progenitor cells (HSPC) under normal and stressed conditions. We show that HSPCs lacking Nras expression are functionally equivalent to normal HSPCs in the adult mouse. Importantly, shRNA-mediated knockdown in human AML cell lines and primary mouse leukemias with oncogenic NRAS/Nras mutations revealed dependence on continued oncogene expression in vitro and in vivo. Next, we interrogated the functional consequences of pharmacologic inhibition of the canonical Ras effector pathways, the mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) pathways,alone and in combination. Recipient mice transplanted with five independent primary mouse AMLs generated by infecting Nras^G12D “knock in” mice with the MOL4070LTR retrovirus (Li et al, Blood 2011; 117:2022) were treated with the allosteric MEK inhibitors PD0325901 (PD901) or trametinib or the PI3K inhibitor GDC-0941. Both MEK inhibitors significantly prolonged survival and reduced proliferation and blast colony formation, but did not induce apoptosis, differentiation, or promote clonal evolution. PI3K inhibition alone was ineffective in vivo and combinations of MEK and PI3K inhibitors were no better than MEK inhibition alone. All mice ultimately succumbed from progressive leukemia. These data, along with observations that Nras is dispensable for normal hematopoiesis, validate oncogenic N-Ras signaling as a therapeutic target in AML and support testing combination regimens that include MEK inhibitors in leukemias harboring NRAS mutations.