Furthermore, it can also help explain the mechanism behind bortezomib resistance in different tumors. Standard of care treatment for PV is phlebotomy supplemented with low-dose aspirin, and for intermediate to high risk patients follow-up hydroxyurea can be prescribed. While these treatment regimens have provided an excellent survival benefit, patients still suffer from reduced quality of life, owing to chronic fatigue, BMN673 pruritis and bone pain. Additionally patients are at significant risk of transformation to hematological malignancies such as AML. Treatment with JAK2 inhibitors can attenuate these symptoms, thus, there is genetic, cell based, and in vivo evidence to suggest a functional role for mutant JAK2 in the pathology of PV, and it is reasonable to predict that targeting the JAK2 protein could have therapeutic benefit in this patient population. In fact, the MPD community has been eagerly anticipating the PI-103 development of JAK inhibitors, and several are currently being tested in clinical trials. Given the broad role of JAK kinases in hematopoiesis, a key challenge will be not only the discovery of high quality targeted agents, but also effective methods of their use, as chronic, profound inhibition would likely be problematic. We have recently reported the discovery of a potent, orally active inhibitor of JAK2. Polycythemia vera is a disease involving biology for which there is a rich history of study. The discovery of the JAKV617F mutation shed light on the mechanism of disease origin and development. However,from the published literatureit appears that while JAK2 certainly plays an important role, other elements also likely contribute to the pathological evolution of PV. What is not knowniswhether inhibitionof the constitutively activated JAK2 mutant, signaling aberrantly downstream of the EPO receptor in erythroid and myeloid progenitor cells, will provide an effective improvement in the treatment of PV patients. To that end, several groups have developed pre-clinical models of PV disease, enabling the development of JAK2 inhibitors for evaluation in the clinic. A major challenge in development of any JAK2 inhibitor that is not selective for the V617F form of the enzyme is the expected mechanism-based toxicity, as JAK2 signaling is essential for many biological processes, within the hematopoeitic compartment and beyond. Chronic, high-level inhibition of JAK2 would almost certainly be intolerable, even if only considering the intended target tissues of the erythroid and myeloid lineage. Thus the dosing schedule of a JAK2 inhibitor is likely to be an important consideration in addition to the intrinsic properties of that inhibitor when considering its potential for successful clinical application. MRLB-11055 is a potent inhibitor of JAK2, however, similar to other described inhibitors of JAK2, it is not selective for JAK2V617F over JAK2WT. Despite this lack of selectivity at the enzyme level, cells that are dependent on JAK2V617F for growth are much more likely to commit to apoptosis in the presence of MRLB-11055 than their WT counterparts. This suggests a potential problem of adverse effects arising from chronic systemic JAK2 inhibition, and set the stage for exploring intermittent dosing in vivo. The pharmacokinetics of MRLB-11055 in mice was such that we were able to achieve sustained target inhibition each dosing cycle with once daily dosing, enabling efficacy studies. As MRLB-11055 was potent against JAK2WT, we were able to demonstrate efficacy in a model where PV-like symptoms, such as erythrocytosis and splenomegaly, could be rapidly generated by treatment of normal C57BL/6 mice with darbepoetin. While an important proof-of-concept for the inhibitor, this model system is preventative.