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Abstract Myelofibrosis (MF) is characterized by the dysfunctional Janus kinase/signal transducers and activators of transcription signaling (JAK/STAT) pathways leading to progressive proliferation of granulocytic and megakaryocytic cells in the bone marrow at the expense of other hematopoietic lineages. Clinical signs of MF include cytopenias, splenomegaly and transformation to acute leukemia. Jakafi® (ruxolitinib, or rux), a JAK2 inhibitor, is a therapeutic for MF and functions to impair the activated mutations that cause the expansion of megakaryocytic precursors. However, JAK2 also transduces signals of the erythropoietin receptor, thrombopoietin receptor, and the granulocyte colony-stimulating factor receptor. Therefore, individuals being treated with rux are susceptible to treatment-associated effects on normal hematopoiesis resulting in thrombocytopenia, neutropenia and anemia. The TGF-β superfamily plays a vital role in the regulation of hematopoiesis; specifically, SMAD 2/3 activation results in cell quiescence and inhibits precursors from progressing through later stages of hematopoiesis. KER-050, a modified ActRIIA extracellular domain fused to the Fc of human IgG1, is designed to inhibit ligands including activin A, activin B, GDF8 and GDF11, that activate SMAD 2/3. In a preclinical study, administration of KER-050 in mice led to upregulation of erythropoiesis by mobilizing early- and late-stage erythroid precursors and facilitating their terminal maturation into red blood cells (RBCs). In a Phase 1 clinical study, administration of KER-050 to healthy volunteers led to sustained increases in RBCs and hemoglobin (HGB) along with increases in platelets. Given the observed effect of KER-050 on increasing RBCs, we evaluated whether treatment with a research form of KER-050 (RKER-050) could reverse rux-associated reductions in RBCs. Additionally, preclinical studies have shown that KER-050 potentially functioned as a muscle anabolic by increasing lean mass in rodents. We first established anemia in C57Bl/6 mice by dosing with rux before administering RKER-050. Anemia was confirmed on study day 37; mice receiving 120 mg/kg rux via oral gavage (PO) BID had significantly lower RBC (-7.4%, p=0.0001), HGB (-4.0%, p=0.002) and hematocrit (HCT; -5.7%, p=0.0006) levels compared to the control group. Treatment with RKER-050 was initiated on study day 41 and mice received 7.5 mg/kg RKER-050 or vehicle intraperitoneally (IP) twice weekly for approximately 14 days. Mice receiving rux alone continued their decline in RBCs and, on day 55, continued to have significant reductions in RBC (-6.7%, p<0.0001), HGB (-6.0, p<0.00001) and HCT (-5.6%, p=0.0002) levels compared to the control group. These findings are consistent with the progressive effect of JAK2 inhibition on suppressing erythrocyte development and production. In contrast, treatment with RKER-050 abrogated the observed rux-associated reductions in RBCs, HGB, and HCT in the rux-RKER-050 cohort with significant observed increases (+15.8%, +12.2%, +11.2%, respectively, all p<0.0001) when compared to the rux-vehicle group. The rux-RKER-050 cohort also had significantly increased body mass, measured between study day 41 and study day 55 versus the rux-vehicle group (+9.9%, p= 0.006, and +0.69%, respectively). These data demonstrate that rux treatment reduced RBCs, HGB, and HCT in mice, and that coadministration of RKER-050 reversed rux-associated reductions in RBC parameters. Therefore, treatment with KER-050 has the potential to mitigate the dose limiting effects of rux and enhance duration of therapy in MF patients. RKER-050 also increased body weight in mice receiving rux through its anabolic effect on muscle, a potential benefit in elderly MF patients. These data support the potential benefit of KER-050 as a monotherapy and in combination with rux in patients with MF and anemia. KER-050 will be assessed in a Phase 2 clinical trial (KER050-MF-301), which we expect to commence in 2021. Disclosures Nathan: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Feigenson: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Lamora: Keros Therapeutics: Current Employment. Tseng: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Fisher: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Seehra: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Lachey: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees.