Ftx-6058 Induces Fetal Hemoglobin Production and Ameliorates Disease Pathology in Sickle Cell Mice
Authors: Matson David , Xie Keqiang , Roth Mark , Stuart Billy , Bruno Paul , Efremov Ivan , Thompson Lorin , Silver Serena , Moxham Christopher 
Published: November 2021

Abstract
Sickle cell disease (SCD) is a genetic disorder of the red blood cells caused by a mutation in the HBB gene, which results in red blood cell sickling, hemolysis, vaso-occlusive crises (VOCs), and other complications. Increasing HbF has the potential to prevent or reduce disease-related pathophysiology, including the risk of recurring events such as hemolysis and VOCs. Individuals with SCD who also have hereditary persistence of fetal hemoglobin (HPFH) and exhibit HbF levels of 25 - 30% are often asymptomatic from their SCD, underscoring the protective effect of increased HbF in SCD. Preclinical results with FTX-6058 demonstrate robust target engagement and corresponding increases in HbF levels up to approximately 40% of total hemoglobin, demonstrating the potential to have a significant impact in people living with SCD.
FTX-6058 is an investigational, potent, and selective oral small-molecule inhibitor of Embryonic Ectoderm Development (EED) that has been demonstrated to induce robust fetal hemoglobin (HbF) protein expression in cell and murine models of SCD. Here, we report the in vivo effects of EED modulation, and subsequent polycomb repressive complex 2 (PRC2) inhibition in the Townes mouse model of Sickle Cell Disease. Compared with untreated and hydroxyurea treated animals, FTX-6058 (QD, 5mg/kg) exhibits potent target engagement as evidenced by ~70% reduction in H3K27me3 levels, the key epigenetic mark catalyzed by PRC2. Consistent with the robust target engagement observed with FTX-6058, a 2 - 3 fold increase in F cells and HbF protein were observed after 13 and 21 days of FTX-6058 treatment, which was pharmacologically superior to the fetal hemoglobin induction observed with 100 mg/kg hydroxyurea (~25% relative increase in F cells and HbF protein). A linear correlation was also observed between F cells and HbF protein production with FTX-6058 treatment, further supporting the robust HbF induction observed. Consistent with SCD disease presentation, Townes model mice have high reticulocyte counts as a result of premature RBC destruction and hemolysis. Townes model mice treated for 21 days with FTX-6058 ameliorated hemolysis as evidenced by decreases in reticulocytes and increases in red blood cells and total hemoglobin levels. Furthermore, FTX-6058 demonstrated the ability to impact inflammatory drivers of disease as evidenced by decreases in neutrophils and white blood cells. FTX-6058 did not have effects on other hematopoietic lineages. FTX-6058 also impacted pathophysiologic symptoms (e.g., splenomegaly) associated with SCD, as evidenced by ~25% reduction in spleen weight. While some trends were detected, no statistically significant changes were observed in hematological parameters or pathophysiologic symptoms with hydroxyurea, the current standard of care in SCD. These results observed with hydroxyurea underscore the need for new, novel therapies in SCD and other hemoglobinopathies.
Taken together, these studies further support the therapeutic rationale of PRC2 modulation in SCD. The fetal hemoglobin induction and effects on hematological parameters and pathophysiologic symptoms observed with FTX-6058 have the potential to translate to meaningful clinical benefits in SCD and other hemoglobinopathies.

Disclosures
Matson: Fulcrum Therapeutics, Inc.: Current Employment. Xie: Fulcrum Therapeutics, Inc.: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Roth: Fulcrum Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Stuart: Fulcrum Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Bruno: Fulcrum Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Efremov: Fulcrum Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Thompson: Fulcrum Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Silver: Fulcrum Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company. Moxham: Fulcrum Therapeutics, Inc.: Current Employment, Current equity holder in publicly-traded company.