Sotatercept, an Activin Receptor IIa Ligand Trap, Acts Through Bone Marrow Accessory Cells to Promote Late-Stage Erythropoiesis and a Rapid Induction of Red Blood Cell Number and Hemoglobin

• Published in: Blood 2012-11, Vol.120 (21), p.372-372
• Main Authors: Carrancio, Soraya, Markovics, Jennifer A, Wong, Piu, Leisten, Jim, Groza, Matthew C, Raymon, Heather K, Heise, Carla, Chopra, Rajesh, Daniel, Tom O, Sung, Victoria
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood.V120.21.372.372

Abstract 372 The regulation of erythropoiesis requires stem cell factor and erythropoietin (EPO) for the proliferation and survival of erythroid progenitor and early precursor cells. While recombinant EPO is widely used for treating various types of anemia, it often lacks efficacy in cases of anemia due to ineffective erythropoiesis in which immature erythroid precursors undergo apoptosis. Thus, there is an need for new therapies to treat the later stages of erythropoiesis. Members of the transforming growth factor beta (TGFβ) superfamily have been studied as potential regulators of erythropoiesis, iron regulation and globin expression. Sotatercept (ACE-011), a recombinant fusion protein consisting of the extracellular domain of the human activin receptor IIA (ActRIIA) linked to the human immunoglobulin G1 (IgG1) Fc domain, is a ligand trap which binds a number of TGFβ superfamily ligands including activin A, activin B, growth differentiation factor-11 (GDF-11) and bone morphogenetic protein-10 (BMP-10). Administration of sotatercept led to substantial increases in red cell number and hemoglobin in human subjects, but the mechanism is not fully understood. We utilized both mouse in vivo and human in vitro models to investigate the mechanism of sotatercept in promoting erythropoiesis. In order to compare the effects of RAP-011 (the murine version of sotatercept) to EPO on red blood cell (RBC) parameters, C57/Bl mice were dosed with RAP-011, EPO or control vehicle. RAP-011-treated mice had a rapid and statistically significant increase in hematocrit, hemoglobin, and RBC number in less than 72-hours. As rapidly as 24 hours after treatment, RAP-011 induced a significant increase in RNA-negative, enucleated cells in the bone marrow (BM). RAP-011 also rapidly increased BM BFU-e and CFU-e erythroid progenitors, while EPO was more effective on spleen-derived progenitors. These data suggest that RAP-011 acts primarily on both bone marrow progenitor cells and late erythroblasts to promote erythropoiesis. In order to investigate the cellular mechanism by which RAP-011 increases red blood cell parameters, we conducted a series of in vitro experiments and found no evidence to support direct effects of RAP-011 on human CD34+ cells assessed in colony formation assays and in erythroid differentiation in liquid culture. As both clinical and pharmacological findings point to a clear role for RAP-011 in stimulating RBC parameters, we hypothesized that RAP-011 effects may be