Rker-050, a Novel Activin Receptor Type II Ligand Trap, Rescued Anemia and Reduced Bone Loss in a Mouse Model of Myelodysplastic Syndromes

Myelodysplastic syndromes (MDS) are a group of hematopoietic stem cell diseases characterized by ineffective hematopoiesis, peripheral blood cytopenias and dysplasias resulting from molecular defects and dysregulated signaling that govern the regulation of hematopoietic stem and progenitor cells (HSPCs). Crosstalk between cells in the osteo-hematopoietic (O-H) niche are necessary for the maintenance and self-renewal of HSPCs. Additionally, MDS patients commonly present with osteopenia and osteoporosis further demonstrating the critical interplay between the O-H cells in the bone marrow (BM). Alterations in the interaction between bone precursors and HSPCs within the BM microenvironment have been linked to the pathophysiology of MDS. Transforming growth factor-beta (TGF-β) superfamily signaling is instrumental in regulating both normal hematopoiesis and bone homeostasis. KER-050 is a modified investigational activin receptor type II ligand trap designed to bind and inhibit select TGF-β superfamily ligands, including activin A, activin B, GDF8 and GDF11, to promote hematopoiesis and bone growth. Preclinical studies have demonstrated that treatment with a research form of KER-050 (RKER-050) increased bone mass and multi-lineage hematopoiesis in healthy mice.

To further investigate the effect of RKER-050 on hematopoietic and bone systems within the MDS disease setting, the NUP98/HOX13 MDS mouse model was utilized. Five-month-old male MDS mice were treated with vehicle (VEH, n=10), or RKER-050 (050, 7.5mg/kg, n=9) once weekly for 6 weeks along with a wildtype vehicle (WT, n=7) control group. To assess changes in hematological parameters, CBCs were measured, bone marrow (BM) and splenocytes were characterized via flow cytometry. To investigate alterations of bone microarchitecture, femurs were assessed using microCT imaging.

MDS-VEH mice showed a reduction in red blood cells (RBC; 30%), hemoglobin (HGB; 19%), hematocrit (HCT; 13%) and reticulocytes (Retic; 16%) compared to WT; all hallmarks of human MDS. Relative to MDS-VEH, MDS-050 showed increased RBC, HGB, HCT and Retic by 25%, 13%, 10% and 39%, respectively. The erythroid Ter119 positive (Ter119+) cell population was slightly reduced in BM and spleen of MDS-VEH mice compared to WT. The BM Ter119+ cells were unchanged in the MDS-050 cohort relative to MDS-VEH; however, the spleen Ter119+ cells were increased similar to the levels in WT mice. Specific erythroid precursor populations were increased in BM and spleen of MDS-VEH mice versus WT, consistent with the ineffective erythropoiesis in this model. Interestingly, erythroid precursor populations were similar between MDS-VEH and MDS-050, yet MDS-050 exhibited recovered RBC production, suggesting an increase in maturation of erythroid precursors with RKER-050 treatment.

Given the importance of the O-H niche in regulating hematopoiesis along with osteoporosis and osteopenia being comorbidities of MDS, bone microarchitecture was also analyzed. In the distal femoral trabecular bone, relative to WT mice, MDS-VEH mice had significant increases in trabecular (Tb) separation (33%) and decreases in Tb number (28%), changes that are consistent with reduced bone strength. Bone volume (34%) and bone volume fraction (26%) decreased but were not statistically significant. MDS-050 treated mice had a significant increase in bone volume (71%), higher bone volume fraction (88%), increased Tb number (44%) and decreased Tb spacing (20%) compared to the MDS-VEH, supporting RKER-050's role in improving bone mass.

Taken together, these results suggest that RKER-050 can mitigate anemia and bone loss in an MDS mouse model, potentially by rebalancing hematopoiesis and bone turnover. In particular, the ability of RKER-050 to improve bone microarchitecture within the trabecular region, where the O-H niche resides, could be an important step in reestablishing healthy blood cell production. Furthermore, exploratory PD data from an ongoing phase 2 clinical trial demonstrate improved blood parameters with KER-050 treatment, including platelet and RBC in MDS patients (Tan et al. 2022 EHA #P776), and changes in bone biomarkers are currently being evaluated in the study. KER-050, thus, represents a potentially promising approach for patients with MDS and other hematological diseases, including myelofibrosis, where ineffective hematopoiesis and defects in bone homeostasis occur.

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