Inhibition of Akt Signaling Alleviates MDS/MPN Driven By KrasD12 or Nf1 Loss

Juvenile and chronic myelomonocytic leukemias (JMML and CMML) are aggressive myeloid malignancies categorized as myelodysplastic syndromes/myeloproliferative neoplasms (MDS/MPN). Chemotherapy has little benefit for MDS/MPN patients, and new therapies are needed. We have used mouse models investigate the potential of signal transduction inhibitors in MDS/MPN, as JMML and CMML are associated with mutations in NRAS , KRAS , PTPN11 , CBL , or NF1 that activate Ras signaling. Conditional Mx1-Cre, Kras LSL-D12 (designated Kras D12 ) mice develop an aggressive and fully penetrant MDS/MPN characterized by leukocytosis, splenomegaly, anemia, and death by 10-16 weeks of age. Mx1-Cre , Nf1 flox/- mice (hereafter Nf1 Δ/- ) undergo conditional loss of Nf1 . These mice also develop MDS/MPN, but the disease is more indolent. We and others have investigated inhibition of effector networks downstream of Ras, such as the Raf/MEK/ERK (MAPK) and phosphotidylinositol-3 kinase (PI3K)/Akt pathways. We previously showed that the MEK inhibitor PD0325901 induced sustained hematologic improvement in both Kras D12 and Nf1 Δ /- mice. We also have reported that the class I PI3K inhibitor GDC-0941 improves hematologic function and prolongs survival in Kras D12 mice. However, GDC-0941 and other PI3K inhibitors attenuate both PI3K/Akt and Raf/MEK/ERK pathways due to effects of PI3K upstream of Ras. Therefore, the benefit from GDC-0941 could have been due to its modulation of Raf/MEK/ERK signaling. Here, we specifically test the importance of Akt signaling in MDS/MPN in Kras D12 and Nf1 mouse models using the allosteric inhibitor MK-2206. This compound binds to the interface of the PH and kinase domains of Akt1, Akt2, and Akt3, and does not inhibit any of 250 other kinases at 1 µM. MK-2206 induced substantial improvement in both Kras D12 and Nf1 Δ /- mice. Mice treated with MK-2206 had pronounced reduction in leukocytosis, reticulocytosis and splenomegaly, increased hemoglobin concentration, and prolonged survival. MK-2206 had no hematologic effects in control WT mice, indicating some selectivity against aberrant hematopoiesis. Importantly, MK-2206 inhibited Akt but not Raf/MEK/ERK or Jak/STAT signaling. This demonstrates that canonical PI3K/Akt signaling plays an important role in Ras-driven MDS/MPN. Furthermore, combined inhibition of MEK and Akt with PD0325901+MK-2206 yielded a greater improvement in splenomegaly than either agent alone in both Kras D12 and Nf1 Δ /- models. Akt has multiple effectors relevant to hematopoiesis and leukemia. Of these, mTOR is of particular interest for targeted cancer therapy. Therefore, we tested the response of Kras D12 mice to rapamycin, a partial inhibitor of mTOR with preferential activity against the mTORC1 complex. Kras D12 mice demonstrated variable responses to rapamycin, with approximately half undergoing a complete and durable hematologic response and the remainder having no response. Together, these studies further implicate PI3K/Akt signaling as a pathogenic effector downstream of Ras in MDS/MPN and support the idea that inhibitors targeting this pathway may have a role in treatment of JMML or CMML. Disclosures No relevant conflicts of interest to declare.