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4236 Anemia is one of the most common blood disorders in several diseases including cancer, heart failure, chronic kidney disease (CKD) and Myeloid Dysplastic Syndromes (MDS) associated with a negative outcome. Administration of recombinant Erythropoietin (EPO) represents the most common treatment for anemia. However, a significant number of people remain hypo or non-responsive to EPO treatment, and in some cases its use has been linked to tumor growth, cardiovascular disease and poorer survival. The members of TGFβ super family of ligands (Activins, GDFs and BMPs) and receptors (Type I and II) regulate more than 500 target genes transcriptionally by Smad phosphorylation and are involved in many cellular functions including cell growth, adhesion, migration, differentiation and apoptosis in a concentration and context dependent manner. Members of the TGFβ family have also demonstrated a role in erythropoiesis. ACE-536, a non-ESA agent is a soluble human Fc fusion chimera of a modified Activin Type IIb receptor with a mutation in its extracellular domain. Surface Plasmon Analysis (Biacore) analysis and cell based reporter assays revealed that this mutation disrupted its binding to Activin A but not to GDF11 or GDF8. ACE-536 acts as a decoy receptor for TGFβ signaling and demonstrated potent increase in red blood cells in all the tested animals (mice, rats and monkeys). Subcutaneous administration of ACE-536 (10mg/kg) to C57BL/6 mice resulted in a significant increase in hematocrit, hemoglobin and red blood cells (RBC) over the TBS treated vehicle group after 4 days. These observations were seen even in the presence of an EPO neutralizing antibody; suggesting that EPO is not directing the initial RBC response to ACE-536 treatment. There were no increase in BFU-E or CFU-E colony formation from bone marrow and spleen after 48hrs treatment with ACE-536 over TBS treated group demonstrating that it does not have effect on erythroid progenitor population. Differentiation profiling of bone marrow and splenic erythroblasts by flow cytometric analysis revealed that ACE-536 promotes maturation of developing erythroblasts. ACE-536 treatment for 72 hours resulted in a decrease in basophilic erythroblasts and an increase in late stage poly, ortho chromatophilic erythroblasts in bone marrow and spleen compared to the TBS treated mice. Treatment of Sprague-Dawley rats with a murine analogue of ACE-536 (RAP-536; 10mg/kg) increased the reticulocyte formation in peripheral blood over vehicle treated group. ACE-536 (10mg/kg) treatment combined with recombinant human EPO (1800 units/kg) for 72 hours increased RBC, hematocrit and hemoglobin by approximately 23% over TBS treated vehicle group and 12% over EPO treatment alone. Consistent with its role in proliferation, EPO increased splenic basophilic erythroblast formation. However, ACE-536 treatment combined with EPO significantly promoted maturation of late stage erythroblasts; demonstrating a novel mechanism during erythroid differentiation. To gain further insights into its mechanism of action, C57BL/6 mice were administered with or without RAP-536 (10mg/ml twice a week) pre treated for a week with neutralizing anti-Activin A (10mg/kg) or ActRIIa (10mg/ml) or ActRIIb (10mg/ml) (does not bind ACE-536) antibodies. Anti-ActRIIa but not anti-Activin A or anti- ActRIIb antibody pre-treatment inhibited the RBC increase by RAP-536 suggesting that ActRIIa or its ligands are necessary for transducing the signal. To summarize, ACE-536 treatment results in a rapid increase in red blood cells by a novel mechanism promoting maturation of late stage erythroblasts. The efficacy of ACE-536 molecule was tested in several acute and chronic anemia animal models including blood loss anemia, chemotherapy induced anemia, chronic kidney disease (5/6 Nephrectomy) and Myeloid Dysplastic Syndrome (MDS) and found that ACE-536 treatment prevents or decreases anemia in all these models. Furthermore, unlike EPO, ACE-536 did not promote tumor progression (in Lewis Lung Carcinoma model) thus offering strong promise as alternate treatments for anemia. Disclosures: Suragani: Acceleron Pharma: Employment. Cadena:Acceleron Pharma: Employment. Mitchell:Acceleron Pharma: Employment. Sako:Acceleron Pharma: Employment. Davies:Acceleron Pharma: Employment. Tomkinson:Acceleron Pharma: Employment. Devine:Acceleron Pharma: Employment. Ucran:Acceleron Pharma: Employment. Grinberg:Acceleron Pharma: Employment. Underwood:Acceleron Pharma: Employment. Pearsall:Acceleron Pharma: Employment. Seehra:Acceleron Pharma: Employment. Kumar:Acceleron Pharma: Employment.

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