Ker-050, an Inhibitor of TGF- β Superfamily Signaling, Promoted Thrombopoiesis and Reversed Immune Thrombocytopenia in a Mouse Model of Disease
Authors: Feigenson Marina , Nathan Remya , Lamora Joshua , Fisher Ffolliott , Tseng Claire C , Seehra Jasbir , Lachey Jenn 
Published: November 2021

Abstract
KER-050 is a modified ActRIIA ligand trap that is designed to inhibit the activity of the TGF-β ligands, including activin A, activin B, GDF8 and GDF11, that act through the SMAD2/3 signaling cascade. Notably, in a Phase 1 clinical study we observed that, in addition to increases in red blood cells and hemoglobin, treatment with KER-050 elicited a robust and sustained increase in platelets (PLTs) in healthy volunteers. While ActRII ligand traps have been shown to increase erythropoiesis in preclinical and clinical studies, their role in thrombopoiesis has not yet been well-elucidated.
A variety of conditions exist where hematopoiesis is impaired and cytopenia persists, including comorbidities associated with aging, diseases causing ineffective hematopoiesis such as myelodysplastic syndrome (MDS) and myelofibrosis (MF), and acute bleeding. Thrombocytopenia can arise from primary or secondary causes due to multiple etiologies and treatments are limited, aiming at treating the root cause of disease or replacing PLTs through transfusion. Therefore, there is an unmet medical need for more targeted treatments to correct thrombocytopenia. Here, in a series of preclinical studies, we investigated the mechanism of action of KER-050 on thrombopoiesis and evaluated its ability to accelerate recovery from acute platelet depletion.
First, we examined how RKER-050 (a research form of KER-050) affected thrombopoiesis under homeostatic conditions. We observed that a single intraperitoneal dose of RKER-050 (10 mg/kg) to 11-week-old mice resulted in a 2-fold increase in PLTs compared to vehicle-treated mice 12 hours after treatment. The timing of the effect is suggestive of a direct effect of RKER-050 on terminal platelet maturation. Additionally, there was a 35% increase in the number of bone marrow (BM) megakaryocyte (Mk) progenitors (Lin -; sca1 -; cKit -; CD41 + cells), demonstrating that RKER-050 affected earlier stages of the PLT formation process. We also evaluated the effect of RKER-050 on the polyploidization of Mks, a hallmark of Mk differentiation. At 24 hours after treatment with RKER-050, there was an increase in BM CD41 + cells with ploidy greater than 16N compared to vehicle-treated mice, demonstrating that RKER-050 treatment resulted in a greater number of Mk that are potentially primed for platelet production. Taken together, these data are consistent with RKER-050 affecting multiple stages of thrombopoiesis in a preclinical model.
We next tested whether RKER-050 affects PLTs in a mouse model of immune thrombocytopenia (IT) where antibodies directed against mouse GPIbα result in acutely reduced PLT numbers. In this model, mice receiving anti-GPIbα had a 25% reduction in PLT number at 4 days post-dose compared to IgG control-treated mice. At this point, the anti-GPIba cohort was divided into receiving either a single dose of vehicle or RKER-050. On day 7 following anti-GPIbα treatment, PLT counts in vehicle-treated mice were 62% lower compared to IgG-control-treated mice. In contrast, the GPIbα-mediated effect on PLT count was stabilized in the anti-GPIbα + RKER-050 group, which had 55% more platelets compared to the anti-GPIba + vehicle group. These data suggest that RKER-050 promoted thrombopoiesis in mice even under conditions when the system is acutely challenged and potentially could promote faster recovery from thrombocytopenia. Additionally, we observed a 25% increase in the number of CD41 + cells in the BM of the RKER-050-treated group compared to the vehicle-treated group at day 10 after PLT depletion, suggesting that under acute thrombocytopenia, RKER-050 treatment promoted differentiation of Mks as a mechanism of the accelerated recovery in platelet-depleted mice.
In summary, our preclinical data demonstrate a potentially novel effect of RKER-050 on thrombopoiesis. RKER-050 treatment resulted in a rapid increase in PLTs, consistent with an effect on terminal maturation. Treatment also increased the number of Mk progenitors and increased the number of polyploid Mks, demonstrating an effect on early stages of thrombopoiesis. These findings support that RKER-050-targeted ligands regulate multiple stages of the thrombopoiesis pathway in mice. Additionally, our data demonstrate that KER-050 has the potential to accelerate the rate of PLT recovery due to acute depletion, and could represent a potential novel treatment option for thrombocytopenia in patients with MDS, MF and IT.

Disclosures
Feigenson: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Nathan: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Lamora: Keros Therapeutics: Current Employment. Fisher: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Tseng: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Seehra: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Lachey: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees.