Etavopivat Increases Arterial Hemoglobin-Oxygen Saturation during Moderate and Severe Hypoxia: A Mechanistic Phase 1 Trial in Healthy Volunteers

Background: Maintaining oxygen delivery during conditions of hypoxia is important, particularly for patients with hemoglobinopathies. Evidence in comparative physiology indicates that high hemoglobin-oxygen (Hb-O₂) affinity serves as a beneficial hematological adaptation to chronic hypoxia exposure. The improved hypoxia tolerance is likely due to an increase in arterial oxygen saturation with high Hb-O₂ affinity. However, the relationship between high Hb-O₂ affinity and hypoxia tolerance in humans is not well understood. Etavopivat is a potent, selective, once-daily, oral activator of the red blood cell (RBC) pyruvate kinase isozyme (PKR), with a half-life of ~13 hours, under investigation for the treatment of sickle cell disease (SCD) and other hematological conditions. Etavopivat has a multimodal mechanism of action in RBCs by decreasing 2,3-diphosphoglycerate (2,3-DPG) levels to increase Hb-O₂ affinity and increasing adenosine triphosphate (ATP) levels to improve RBC health. In Phase 1 trials etavopivat increased Hb-O₂ affinity in healthy volunteers (HVs; Forsyth et al. Clin Pharmacol Drug Dev 2022) and patients with SCD (Saraf et al. Blood Adv 2024) in room air.

Aim: To investigate arterial oxygenation changes with etavopivat under conditions of moderate and severe hypoxia in HVs.

Methods: HVs visited the laboratory for assessments at baseline and following 7 days of etavopivat exposure (400 mg once daily). Participants completed rhythmic handgrip exercises (rest, 10% and 20% maximum voluntary contraction [MVC]) during graded hypoxic exposure, breathing room air (21% O₂), moderate hypoxic gas (15% O₂), and severe hypoxic gas (10% O₂), resulting in acute duration (~12 minutes) of exposure to each level of hypoxia. Hb-O₂ affinity was assessed by partial pressure of oxygen at 50% Hb saturation (P₅₀), measured using dual-wavelength spectrophotometry at a standardized pH (~7.4) and temperature (37 ºC). Arterial catheterization was used to collect samples for fractional oxygen saturation (FO₂Hb) and Hb and to measure arterial blood pressure. Blood gases were analyzed after 5 minutes of resting exposure to each inspirate, then ~2.5 minutes after commencing each of the 3-minute handgrip exercises (10% followed by 20% MVC). Liquid chromatography-tandem mass spectrometry was used to evaluate 2,3-DPG and ATP levels in whole blood. Data for FO₂Hb and heart rate were analyzed using separate repeated measures 2 (treatment) x 3 (intensity) ANOVA for each inspirate and all other hematological data pre vs post etavopivat were analyzed using paired-sample t-tests. Statistical significance was set at P<0.05. Data presented are mean ± standard deviation.

Results: Sixteen HVs (five women) aged 32 ±8 years were enrolled. After 7 days of oral once-daily 400 mg etavopivat, P₅₀ decreased from 28.6 ±1.6 at baseline to 24.8 ±1.3 mmHg (P<0.001). Hb concentration remained unchanged from baseline (14.6 ±1.1 g/dL), post etavopivat (14.5 ±1.1 g/dL; P>0.05). FO₂Hb at rest decreased from 96.0% ±0.6 under normal oxygen conditions to 74.6% ±6.7 during severe hypoxic exposure; however, this decrease in FO₂Hb was mitigated post etavopivat exposure (79.7% ±5.2, P<0.001). A similar mitigation of the FO₂Hb decrease was observed during the handgrip exercise (20% MVC, P<0.001). Concentrations of 2,3-DPG decreased and RBC ATP increased following etavopivat exposure during both normoxia and moderate hypoxia (P<0.001). Mean arterial pressure was lower post etavopivat exposure under normoxia, but not during moderate or severe hypoxia. Heart rate showed no change post etavopivat exposure during normoxia and hypoxia (P>0.50). Etavopivat was well tolerated with no unexpected adverse events reported.

Conclusions: Etavopivat enhanced FO₂Hb during brief hypoxic episodes with an apparent relationship to hypoxia severity for which the greatest improvements were observed at more severe hypoxic exposure. In SCD, high Hb-O₂ affinity is beneficial for maintaining Hb ‘R’ state, thereby reducing the risk of polymerization and associated sickling. Increased Hb-O₂ affinity, together with maintenance of a high FO₂Hb under hypoxic conditions, demonstrates etavopivat’s ability to promote oxygen exchange and subsequent delivery. Together with projected improvement of RBC health, also achieved via PKR activation, this indicates a potential clinical benefit with etavopivat for hemoglobinopathies and beyond.