Rker-050, a Modified Activin Receptor Type Iia Ligand Trap, Regulates Osteogenic Lineage, Complementing the Erythroid Response and Rebalancing the Osteo-Hematopoietic Niche

Background: The osteo-hematopoietic niche (OHN) supports hematopoiesis through interactions between hematopoietic stem cells (HSCs) and stromal cells, such as multipotent stromal cells (BMSCs) and osteoblasts. growth factor (TGF)-β superfamily ligands, including activins and growth differentiation factors (GDFs), regulate not only hematopoiesis but also bone metabolism. In myelodysplastic syndromes (MDS), dysregulation of TGF-β superfamily signaling leads to ineffective hematopoiesis and increased risk of osteoporosis. Elritercept (KER-050), a modified activin receptor type IIA ligand trap, inhibits SMAD2/3 signaling by binding to activins and GDFs. In an ongoing clinical trial (NCT04419649), elritercept has been shown to improve ineffective hematopoiesis and increase markers of bone formation in participants with lower-risk MDS.

Aim: To evaluate the effects of RKER-050, a research version of elritercept, on erythropoiesis and multi-lineage differentiation of BMSCs in steady-state wild-type mice.

Methods: 9-week-old C57BL/6 male mice were treated twice weekly with intraperitoneal (IP) injection of vehicle or 10 mg/kg RKER-050 for 3 weeks. Complete blood count and serum collection were performed at the end of the study. Bone marrow (BM) from RKER-050-treated and vehicle control mice was used for flow cytometry and colony-forming unit assays such as alkaline phosphatase-stained colonies (CFU-ALP) for osteoblast lineage specification, von Kossa-stained colonies (CFU-Ob) for osteoblast differentiation, and Oil Red O-stained colonies (CFU-ADP) for adipocyte differentiation. Isolated BMSCs were used for gene expression analysis of osteoblast and adipocyte lineage differentiation.

Results: RKER-050 treatment significantly increased RBCs (+21%), hemoglobin (+16%), hematocrit (+16%), and reticulocytes (+21%) compared to the vehicle control. RKER-050 treatment significantly increased BM osteoblast (+81%) and BMSC (+23%) populations compared to the vehicle control. In the colony-forming efficiency assays, RKER-050 treatment did not change colony-forming unit fibroblast (CFU-F) numbers compared to the vehicle control, indicating that the BMSC population in the BM was normal. RKER-050 treatment significantly increased CFU-ALP (52%) compared to the vehicle control, indicating enhanced osteoblast lineage specification of BMSCs. To test if the change in osteoblast specification was functional, CFU-F colonies were differentiated into osteoblasts and adipocytes. RKER-050 treatment significantly increased the number of CFU-Ob (+36%) compared to the vehicle control. Conversely, the RKER-050 treatment did not significantly change the number of CFU-ADP compared to the vehicle control. To test if RKER-050 directly promoted differentiation, BMSCs from mice treated with RKER-050 and vehicle control were differentiated into osteoblasts and adipocytes. RKER-050 treatment significantly increased osteoblast marker gene expression, osterix (Sp7), and osteocalcin (Ocn), but no significant changes were seen in adipocyte marker gene expression compared to the vehicle control. These data suggest that RKER-050 can selectively enhance osteoblast differentiation without impacting adipocyte differentiation.

Summary/Conclusion: RKER-050 selectively modulates both erythropoiesis and osteolineage cells simultaneously, demonstrating the potential to restore OHN balance. This dual-action mechanism can enhance critical cellular crosstalk between osteoblasts and hematopoietic stem and progenitor cells, potentially improving the supportive microenvironment for blood cell production. This selective mechanism suggests that elritercept could potentially restore equilibrium in patients with hematological disorders by improving hematopoiesis and promoting bone anabolism.

Dills:Keros Therapeutics: Current Employment. Lerner:Keros Therapeutics: Current Employment. Seehra:Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees. Lachey:Keros Therapeutics: Consultancy, Current equity holder in publicly-traded company. Cadena:Keros Therapeutics: Current Employment.