RAP-536 Promotes Terminal Erythroid Differentiation and Reduces Anemia in a Murine Model of Myelodysplastic Syndromes
Authors: Suragani Rajasekhar NVS , Li Robert , Sako Dianne , Grinberg Asya , Pearsall R. Scott , Kumar Ravindra 
Published: October 2019

Abstract
Abstract 3796
Myelodysplastic syndromes (MDS) are a group of hematopoietic stem cell disorders characterized by peripheral blood cytopenias such as anemia, neutropenia or thrombocytopenia. Ineffective erythropoiesis due to increased proliferation and abortive maturation of precursors leads to severe anemia, the most common cytopenia observed in MDS syndromes. Despite elevated erythropoietin (EPO) and erythroid hyperplasia, MDS patients are often given recombinant EPO therapy to stimulate erythropoiesis. However, only a small proportion of patients respond to EPO therapy. Frequent blood transfusions as supportive care result in iron overloading and recently iron overloading is also linked to enhanced progression to AML. Therefore, alternative therapies are necessary to treat anemia in MDS patients.
Signaling by members of the TGFβ superfamily are known regulators of erythropoiesis. We developed ACE-536, a ligand trap consisting of a modified activin receptor Type IIB extracellular domain linked to a human Fc domain. In vitro assays revealed that ACE-536 inhibits smad 2/3 ligands of the signaling pathway but not smad 1/5/8 ligands. Dose dependent studies using ACE-536 in mice, rats and monkeys revealed that ACE-536 treatment resulted in increased red blood parameters but did not affect other cell types. These data suggests that ACE-536 inhibits smad 2/3 phosphorylation modulating the expression of downstream genes involved in erythroid development pathway. BFU-E and CFU-E colony formation assays from bone marrow and spleen in mice following ACE-536 treatment revealed that ACE-536 did not affect the proliferation stages of erythropoiesis. In mice, terminal erythroid differentiation analysis by flow cytometry at 72hrs following RAP-536 (10mg/kg) treatment demonstrated decreased basophilic and increased ortho- and poly-chromatophilic erythroblasts and reticulocytes compared to VEH treatment. Cell cycle analysis of bone marrow and splenic erythroblasts counterstained with BrdU and 7-AAD after RAP-536 (10mg/kg, for 24 hours) or VEH treatment to EPO pre-treated (1500 units/kg, for 40 hours) mice (N=5/group) revealed that EPO+RAP-536 treatment resulted in significant decrease in S-phase and increase in G1/G2-phases of cell cycle compared to EPO+VEH treatment. In addition, EPO+RAP-536 treatment resulted in a greater increase in RBC parameters than either of the treatments alone. Together, these results demonstrate that ACE-536 increases red blood cell formation by promoting maturation of late stage erythroblasts.
We then investigated the effect of ACE-536 on anemia in NUP98-HOXD13 (NHD13) transgenic murine model of MDS. NHD13 mice develop anemia, neutropenia and lymphopenia, with normal or hyper cellular bone marrow. A Majority of the mice die by 14 months due to severe pancytopenia or progression to acute myeloid leukemia. In this study, mice were divided into three groups based on age. Early (∼4 months old), mid (∼8 months old) and late stage (∼10 months) groups were randomized and dosed with either RAP-536 at 10 mg/kg or VEH twice per week for 6–8 weeks. NHD13 mice in each group had severe anemia characterized by reduced RBC, Hemoglobin and HCT and compared to wild-type littermates prior to treatment. Treatment of RAP-536 for 6–8 weeks significantly increased RBC parameters and reversed anemia at all stages. Peripheral blood smear analysis revealed no indication of increased leukemic progression due to RAP-536 treatment. Cell differential and flow cytometric evaluation of erythroid precursors from bone marrow demonstrated decreased erythroid precursors and hyperplasia after RAP-536 treatment compared to vehicle treated control.
Our data demonstrate that RAP-536 can increase hematology parameters by enhancing maturation of terminally differentiated red blood cells. We have shown RAP-536 corrects ineffective erythropoiesis, decreases erythroid hyperplasia and normalizes myeloid: erythroid ratios without enhanced progression to AML in a murine MDS model. Therefore ACE-536 may represent a novel treatment for anemia associated with MDS, particularly in patients that are refractory to EPO therapy. ACE-536 has completed Phase I clinical trials in healthy human volunteers and Phase II study in MDS patients is planned.

Disclosures:
Suragani: Acceleron Pharma Inc: Employment, Equity Ownership. Li:Acceleron Pharma Inc: Employment, Equity Ownership. Sako:Acceleron Pharma Inc: Employment, Equity Ownership. Grinberg:Acceleron Pharma Inc: Employment, Equity Ownership. Pearsall:Acceleron Pharma Inc: Employment, Equity Ownership. Kumar:Acceleron Pharma Inc: Employment, Equity Ownership.