Impact of Pharmacologically Left Shifting the Oxygen-Hemoglobin Dissociation Curve on Arterial Blood Gases and Pulmonary Gas Exchange During Maximal Exercise in Hypoxia

The dependence of maximum oxygen uptake and utilization (V̇O2 max) on hemoglobin-oxygen affinity and altitude

Oxygen transport from the lungs to peripheral tissue is dependent on the affinity of hemoglobin for oxygen. Recent experimental data have suggested that the maximum human capacity for oxygen uptake and utilization (V̇O2 max) at sea level and altitude (~3000 m) is sensitive to alterations in hemoglobin-oxygen affinity. However, the effect of such alterations on V̇O2 max at extreme altitudes remains largely unknown due to the rarity of mutations affecting hemoglobin-oxygen affinity. This work uses a mathematical model that couples pulmonary oxygen uptake with systemic oxygen utilization under conditions of high metabolic demand to investigate the effect of hemoglobin-oxygen affinity on V̇O2 max as a function of altitude. The model includes the effects of both diffusive and convective limitations on oxygen transport. Pulmonary oxygen uptake is calculated using a spatially-distributed model that accounts for the effects of hematocrit and hemoglobin-oxygen affinity. Systemic oxygen utilization is calculated assuming Michaelis-Menten kinetics. The pulmonary and systemic model components are solved iteratively to compute predicted arterial and venous oxygen levels. Values of V̇O2 max are predicted for several values of hemoglobin-oxygen affinity and hemoglobin concentration based on data from humans with hemoglobin mutations. The model predicts that increased hemoglobin-oxygen affinity leads to increased V̇O2 max at altitudes above ~4500 m.

Effects of GBT1118, a voxelotor analog, on intestinal pathophysiology in sickle cell disease

Voxelotor is an allosteric haemoglobin (Hb) modulator that binds covalently and reversibly to Hb alpha chain to facilitate improved Hb-O₂ affinity and arterial oxygen. It, therefore, reduces the susceptibility of erythrocytes carrying Haemoglobin S to sickle. In this study, we have used GBT1118, an analog of voxelotor, to treat male Townes sickle cell disease (SCD) mice to investigate whether the Hb modulator could attenuate the intestinal pathophysiologic changes associated with SCD. Compared with mice fed with control chow, GBT1118-treated mice showed improvement in the intestinal pathophysiology. These mice exhibited improved small intestinal barrier functions, reduced intestinal microbial density, reduced enterocyte injury, lower serum lipopolysaccharides and smaller spleens. These improvements were observed after only 3 weeks of GBT1118 treatment. Benefits were also observed after experimentally-induced vaso-occlusive crisis (VOC). Recovery from the VOC-induced changes was faster in mice that were treated with GBT1118. The improved small intestinal barrier function was associated with higher expression of genes encoding enterocyte E-cadherin, JAM-A, ZO-1, MUC-2 and occludin while the lower intestinal microbial density associated with higher expression of genes encoding the antimicrobial peptides defensin-α 1 and defensin-α 4. Our findings provide the evidence to support the beneficial effects of GBT1118 in SCD-related intestinal pathophysiology.

A phase 1 dose escalation study of the pyruvate kinase activator mitapivat (AG-348) in sickle cell disease

Polymerization of deoxygenated hemoglobin S underlies the pathophysiology of sickle cell disease (SCD). In activating red blood cell pyruvate kinase and glycolysis, mitapivat (AG-348) increases adenosine triphosphate (ATP) levels and decreases the 2,3-diphosphoglycerate (2,3-DPG) concentration, an upstream precursor in glycolysis. Both changes have therapeutic potential for patients with SCD. Here, we evaluated the safety and tolerability of multiple ascending doses of mitapivat in adults with SCD with no recent blood transfusions or changes in hydroxyurea or l-glutamine therapy. Seventeen subjects were enrolled; 1 subject was withdrawn shortly after starting the study. Sixteen subjects completed 3 ascending dose levels of mitapivat (5, 20, and 50 mg, twice daily [BID]) for 2 weeks each; following a protocol amendment, the dose was escalated to 100 mg BID in 9 subjects. Mitapivat was well tolerated at all dose levels, with the most common treatment-emergent adverse events (AEs) being insomnia, headache, and hypertension. Six serious AEs (SAEs) included 4 vaso-occlusive crises (VOCs), non-VOC-related shoulder pain, and a preexisting pulmonary embolism. Two VOCs occurred during drug taper and were possibly drug related; no other SAEs were drug related. Mean hemoglobin increase at the 50 mg BID dose level was 1.2 g/dL, with 9 of 16 (56.3%) patients achieving a hemoglobin response of a ≥1 g/dL increase compared with baseline. Mean reductions in hemolytic markers and dose-dependent decreases in 2,3-DPG and increases in ATP were also observed. This study provides proof of concept that mitapivat has disease-modifying potential in patients with SCD. This trial was registered at www.clinicaltrials.gov as #NCT04000165.

Revisiting anemia in sickle cell disease and finding the balance with therapeutic approaches

Chronic hemolytic anemia and intermittent acute pain episodes are the 2 hallmark characteristics of sickle cell disease (SCD). Anemia in SCD not only signals a reduction of red cell mass and oxygen delivery, but also ongoing red cell breakdown and release of cell-free hemoglobin, which together contribute to a number of pathophysiological responses and play a key role in the pathogenesis of cumulative multiorgan damage. However, although anemia is clearly associated with many detrimental outcomes, it may also have an advantage in SCD in lowering risks of potential viscosity-related complications. Until recently, clinical drug development for SCD has predominantly targeted a reduction in the frequency of vaso-occlusive crises as an endpoint, but increasingly, more attention is being directed toward addressing the contribution of chronic anemia to poor outcomes in SCD. This article aims to explore the complex pathophysiology and mechanisms of anemia in SCD, as well as the need to balance the benefits of raising hemoglobin levels with the potential risks of increasing blood viscosity, in the context of the current therapeutic landscape for anemia in SCD.

Altitude acclimatization, hemoglobin-oxygen affinity, and circulatory oxygen transport in hypoxia

In mammals and other air-breathing vertebrates that live at high altitude, adjustments in convective O₂ transport via changes in blood hemoglobin (Hb) content and/or Hb-O₂ affinity can potentially mitigate the effects of arterial hypoxemia. However, there are conflicting views about the optimal values of such traits in hypoxia, partly due to the intriguing observation that hypoxia-induced acclimatization responses in humans and other predominantly lowland mammals are frequently not aligned in the same direction as evolved phenotypic changes in high-altitude natives. Here we review relevant theoretical and empirical results and we highlight experimental studies of rodents and humans that provide insights into the combination of hematological changes that help attenuate the decline in aerobic performance in hypoxia. For a given severity of hypoxia, experimental results suggest that optimal values for hematological traits are conditional on the states of other interrelated phenotypes that govern different steps in the O₂-transport pathway.

Influence of High Hemoglobin-Oxygen Affinity on Humans During Hypoxia

Humans elicit a robust series of physiological responses to maintain adequate oxygen delivery during hypoxia, including a transient reduction in hemoglobin-oxygen (Hb-O₂) affinity. However, high Hb-O₂ affinity has been identified as a beneficial adaptation in several species that have been exposed to high altitude for generations. The observed differences in Hb-O₂ affinity between humans and species adapted to high altitude pose a central question: is higher or lower Hb-O₂ affinity in humans more advantageous when O₂ availability is limited? Humans with genetic mutations in hemoglobin structure resulting in high Hb-O₂ affinity have shown attenuated cardiorespiratory adjustments during hypoxia both at rest and during exercise, providing unique insight into this central question. Therefore, the purpose of this review is to examine the influence of high Hb-O₂ affinity during hypoxia through comparison of cardiovascular and respiratory adjustments elicited by humans with high Hb-O₂ affinity compared to those with normal Hb-O₂ affinity.