Increased Hemoglobin Affinity for Oxygen with GBT021601 Reduces Pulmonary Hypertension and Lung Vascular Inflammation in a Murine Model of Sickle Cell Disease

Background: Sickle cell disease (SCD) is caused by a single nucleotide change in the β-globin gene coding sequence that results in the production of hemoglobin S (HbS). Under conditions of red blood cell (RBC) deoxygenation, HbS polymerizes and leads to RBC sickling, resulting in hemolysis, anemia, diminished microvascular blood flow, vascular endothelial cell activation, vaso-occlusion, and ischemia. Despite improvements in treatment options, SCD remains a debilitating condition, and most patients experience progressive end-organ damage with associated complications and repeated hospitalizations. Pulmonary hypertension (PH), which is associated with increased mortality, is a common complication in patients with SCD, with nearly 10% of the SCD population reporting PH. PH in patients with SCD is multifactorial and may be caused by hemolysis, nitric oxide scavenging, and/or tissue hypoxia. Pharmacological increase of the concentration of oxygenated HbS (oxyHbS) in RBCs by voxelotor, a first-generation HbS polymerization inhibitor, reduces RBC sickling and preserves oxygen (O₂) delivery by reducing anemia and improving blood rheology. Limited data are currently available on whether inhibiting HbS polymerization by increasing oxyHbS concentration may improve end-organ damage and associated complications, including PH.

Objective: To examine the effects of GBT021601, a small-molecule, investigational, second-generation HbS polymerization inhibitor, on hemoglobin (Hb)-O₂ affinity, PH, right ventricular dysfunction, and lung inflammation in a murine model of SCD.

Methods: Townes HbSS mice treated with GBT021601 (75 mg/kg orally once daily for 14 days) were exposed to acute hypoxia (8% O₂) for 1 hour and reoxygenated for 72 hours. Untreated HbAA Townes mice were used as controls. Lung inflammation was assessed in terms of accumulation of inflammatory cells, activation of nuclear factor kappa B subunit 1, and expression of intercellular adhesion molecule 1 and vascular cell adhesion molecule 1. To evaluate the impact of GBT021601 treatment on Hb-O₂ affinity and anemia, p50 (partial O₂ pressure at which Hb is 50% saturated with O₂) and hematocrit levels were measured in SCD mice. In vivo pulmonary hemodynamic measurements included pulmonary artery pressure and right atrial pressure, whereas right ventricular hypertrophy was assessed using the Fulton index, which is the ratio of right ventricular weight to the weight of the left ventricle plus septum.

Results: The total number of inflammatory cells and the specific numbers of lymphocytes, macrophages, and neutrophils were lower in untreated HbAA Townes mice and in HbSS Townes mice treated with GBT021601 compared with vehicle-dosed HbSS mice (Figure, panel A). Chronic treatment with GBT021601 increased Hb-O₂ affinity, reducing the p50 in SCD mice from 32 mm Hg to 14 mm Hg. The treatment significantly reduced anemia, increasing hematocrit from 27.2% in the vehicle-dosed HbSS mice to 38.5% in the mice treated with GBT021601. Chronic GBT021601 treatment in HbSS mice also reduced pulmonary artery and right atrial pressure compared with those in untreated HbSS mice. Additionally, GBT021601 reduced right ventricular hypertrophy in HbSS mice, as evidenced by a reduction in the Fulton ratio (Figure, panel B).

Conclusions: GBT021601 reduced PH, right ventricular dysfunction, and lung inflammation in a murine model of SCD. Our findings suggest that reducing RBC sickling, anemia, and hemolysis by increasing HbS affinity for O₂ plays a critical role in modulating the immune system in PH and lung vasculature. These findings highlight the potential therapeutic efficacy of GBT021601 based on its ability to increase HbS affinity for O₂ and prevent HbS polymerization and RBC sickling to reduce PH in SCD.

Funding: Global Blood Therapeutics.

Dufu:Global Blood Therapeutics: Current Employment, Current equity holder in publicly-traded company. Oksenberg:Global Blood Therapeutics: Current equity holder in publicly-traded company, Ended employment in the past 24 months. Cathers:Global Blood Therapeutics: Current Employment, Current equity holder in publicly-traded company. Munoz:Global Blood Therapeutics: Research Funding. Muller:Global Blood Therapeutics: Research Funding. Pedro Cabrales:Global Blood Therapeutics: Research Funding.