Navani, Sunil

Background: During acute hypoxia, Thehuman brainemploys complex regulatory mechanisms to compensate for high O 2 demand and low O 2 blood saturation. High hemoglobin-oxygen (Hb-O 2 ) affinity improves arterial oxygen saturation (S a O 2 ) in low oxygen environments providing a potential physiologic benefit in humans. However, the impact of high Hb-O 2 affinity on cerebral blood flow (CBF) and cerebral tissue oxygenation (S t O 2 ) has yet to be examined. This work aimed to determine the influence of an acute increase in Hb-O 2 affinity from Etavopivat, a selective red blood cell pyruvate kinase activator, on CBF regulation and S t O 2 during handgrip exercise in normoxia and hypoxia. Methods: 16 healthy adults (31±8 yr, 5 females) completed two visits; before and after Etavopivat (400 mg daily, 7 days) administration,. Each visit included graded handgrip (~3 min stages each) exercise (rest, 10% maximal voluntary contraction (MVC), 20% MVC) while participants inspired normoxic (NORM, 21% O 2 ), moderate (MH, 15% O 2 ), and severe (SH, 10% O 2 ) hypoxic gas mixtures. Hb-O 2 binding affinity (P 50 ), was measured using dual-wavelength spectrophotometry (pH~7.4 and 37ºC). Blood sampling through catheterization of the non-exercising arm measured S a O 2 . CBF was approximated by the peak middle cerebral artery velocity, measured with a 2MHz transcranial doppler ultrasound through the transtemporal window. S t O 2 was measured using frequency-domain near infrared spectroscopy of the frontal cortex. CBF and S t O 2 between visits were compared using separate two-way RM ANOVA models to determine the effects of Etavopivat [before (baseline) or after 7d Etavopivat exposure (post)] and exercise intensity [rest, 10% MVC, 20% MVC] for each inspirate. Results: P 50 was reduced after Etavopivat exposure (Baseline: 28.6±1.6 mmHg, Etavo: 24.8±1.3, P <0.001) without impacting hematocrit ( P =0.97). S a O 2 was higher following Etavopivat exposure during each inspirate ( P <0.001 for each). There was no significant effect of handgrip exercise on CBF or S t O 2 within any inspirate. Cerebral S t O 2 was higher during each inspirate (all P <0.002)– NORM (Baseline: 65.3±12.3%, Post: 68.8±9.8%), MH (Baseline: 61.0±11.7 %, Post: 66.1±10.5%) and SH (Baseline: 47.7±13.5%, Post: 56.6±11.5%). In addition, CBF was increased during each inspirate (all P <0.001)– NORM (Baseline: 62.2±12.2 cm/s, Post: 69.2±12.5 cm/s), MH (Baseline: 60.8±13.1 cm/s, Post: 67.9±12.3 cm/s) and SH (Baseline: 69.8±16.3 cm/s, Post: 79.2±18.0 cm/s). Conclusion: The acute pharmacological induction of high Hb-O 2 affinity leads to increased CBF and cerebral S t O 2 during acute hypoxic exposure in conditions of mild muscular exertion. Whether the improvement of cerebral S t O 2 is associated increased S a O 2 or CBF (or both) is unclear. These findings are the first to investigate the complex determinants of cerebral S t O 2 as a function of an acute increase in Hb-O 2 affinity. These improvements in S t O 2 during hypoxic exposure may lead to a host of theoretical benefits such as improved hypoxia tolerance, and mitigated decrements in cognitive function during hypoxic exposure. This study was funded by Forma Therapeutics Inc, which was acquired by Novo Nordisk on October 14, 2022, Novo Nordisk A/S. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.

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&lt;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&lt;0.001). Hb concentration remained unchanged from baseline (14.6 ±1.1 g/dL), post etavopivat (14.5 ±1.1 g/dL; P&gt;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&lt;0.001). A similar mitigation of the FO2Hb decrease was observed during the handgrip exercise (20% MVC, P&lt;0.001). Concentrations of 2,3-DPG decreased and RBC ATP increased following etavopivat exposure during both normoxia and moderate hypoxia (P&lt;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&gt;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.