In Vivo Characterization of Ftx-6058, a Novel Small Molecular Fetal Hemoglobin Inducer for Sickle Cell Disease

Sickle cell disease (SCD) results from genetic mutation in the β-globin gene encoding a subunit of the adult form of hemoglobin (HbA), leading to red blood cell (RBC) deformation and disease pathology. It has been demonstrated that reactivation of the fetal ortholog of the hemoglobin beta subunit, HBγ (also referred to as HBG proteins), can prevent or reduce disease-related pathophysiology. In SCD, the presence of HBG protein in hemoglobin tetramers prevents sickle hemoglobin polymerization under deoxygenated conditions and therefore may be of therapeutic benefit in SCD. FTX 6058, a novel orally bioavailable small molecule, is in development for the treatment of sickle cell disease (SCD) by Fulcrum Therapeutics. FTX-6058 was demonstrated to inhibit the novel biological target and elevate the expression of HBγ, resulting in induction of fetal hemoglobin (HbF) tetramer in differentiated human primary CD34⁺ cells.

The in vivo target engagement (TE) and pharmacologic effects of FTX-6058 were characterized in wild-type CD-1 mice and humanized Townes SCD mice, with TE also confirmed in non-human primates. In CD-1 mice, once-daily (QD) FTX-6058 oral administration induced TE in a time- and dose-dependent manner and most markedly in erythroid lineage (Ter119⁺) cells derived from bone marrow, the putative therapeutic compartment, and increased transcript levels of Hbb-bh1, a murine embryonic hemoglobin surrogate for human HBG gene. Steady state TE in circulating monocytes, following repeated QD FTX-6058 administration, correlated well with that in bone marrow-derived erythroid cells, suggesting peripheral monocytes as a suitable surrogate for assessing erythroid TE activity in Fulcrum's Phase 1 study. In non-human primate cynomolgus monkeys, QD oral dosing of FTX-6058 as early as for 7 days induced robust and comparable TE in bone marrow derived CD34+ erythroid progenitors and circulating monocytes, further supporting the use of monocytes to assess TE in bone marrow. Mouse data also provided evidence of the reversibility of TE effects once dosing is stopped.

In repeat-dose studies in the humanized Townes SCD mouse model, FTX-6058 was superior to standard of care hydroxyurea as measured by human HBG1 transgene induction and increased %F-cells and HBG1 protein levels. Furthermore, the induction of %F cells was sustainable for several days after dosing cessation.

These in vivo studies have demonstrated that FTX-6058 engages its novel biological target in multiple preclinical models and induces HbF expression at plasma concentrations likely to b e readily achievable in clinic, and peripheral monocytes is a suitable surrogate for assessing TE in bone marrow erythroid cells, which could be beneficial to patients with SCD.