Quinine protects pyruvate-kinase deficient red cells from dehydration

Activation of pyruvate kinase as therapeutic option for rare hemolytic anemias: Shedding new light on an old enzyme

Novel developments in therapies for various hereditary hemolytic anemias reflect the pivotal role of pyruvate kinase (PK), a key enzyme of glycolysis, in red blood cell (RBC) health. Without PK catalyzing one of the final steps of the Embden-Meyerhof pathway, there is no net yield of adenosine triphosphate (ATP) during glycolysis, the sole source of energy production required for proper RBC function and survival. In hereditary hemolytic anemias, RBC health is compromised and therefore lifespan is shortened. Although our knowledge on glycolysis in general and PK function in particular is solid, recent advances in genetic, molecular, biochemical, and metabolic aspects of hereditary anemias have improved our understanding of these diseases. These advances provide a rationale for targeting PK as therapeutic option in hereditary hemolytic anemias other than PK deficiency. This review summarizes the knowledge, rationale, (pre)clinical trials, and future advances of PK activators for this important group of rare diseases.

AG-348 (Mitapivat), an allosteric activator of red blood cell pyruvate kinase, increases enzymatic activity, protein stability, and ATP levels over a broad range of PKLR genotypes

Pyruvate kinase (PK) deficiency is a rare hereditary disorder affecting red blood cell (RBC) glycolysis, causing changes in metabolism including a deficiency in adenosine triphosphate (ATP). This affects red cell homeostasis, promoting premature removal of RBC from the circulation. In this study, we characterized and evaluated the effect of AG-348, an allosteric activator of PK that is currently in clinical trials for treatment of PK deficiency, on RBC and erythroid precursors from PK-deficient patients. In 15 patients, ex vivo treatment with AG-348 resulted in increased enzymatic activity in all patients' cells after 24 hours (h) (mean increase: 1.8-fold; range: 1.2-3.4). ATP levels increased (mean increase: 1.5-fold; range: 1.0-2.2) similar to control cells (mean increase: 1.6-fold; range: 1.4-1.8). Generally, PK thermostability was strongly reduced in PK-deficient RBC. Ex vivo treatment with AG-348 increased residual activity from 1.4- to >10-fold more than residual activity of vehicle-treated samples. Protein analyses suggest that a sufficient level of PK protein is required for cells to respond to AG- 348 treatment ex vivo, as treatment effects were minimal in patient cells with very low or undetectable levels of PK-R. In half of the patients, ex vivo treatment with AG-348 was associated with an increase in RBC deformability. These data support the hypothesis that drug intervention with AG- 348 effectively up-regulates PK enzymatic activity and increases stability in PK-deficient RBC over a broad range of PKLR genotypes. The concomitant increase in ATP levels suggests that glycolytic pathway activity may be restored. AG-348 treatment may represent an attractive way to correct the underlying pathologies of PK deficiency. (AG-348 is currently in clinical trials for the treatment of PK deficiency. Registered at clinicaltrials.gov identifiers: NCT02476916, NCT03853798, NCT03548220, NCT03559699).

AG-348 enhances pyruvate kinase activity in red blood cells from patients with pyruvate kinase deficiency

Pyruvate kinase (PK) deficiency is a rare genetic disease that causes chronic hemolytic anemia. There are currently no targeted therapies for PK deficiency. Here, we describe the identification and characterization of AG-348, an allosteric activator of PK that is currently in clinical trials for the treatment of PK deficiency. We demonstrate that AG-348 can increase the activity of wild-type and mutant PK enzymes in biochemical assays and in patient red blood cells treated ex vivo. These data illustrate the potential for AG-348 to restore the glycolytic pathway activity in patients with PK deficiency and ultimately lead to clinical benefit.

A fit-for-purpose LC-MS/MS method for the simultaneous quantitation of ATP and 2,3-DPG in human K2EDTA whole blood

Many hemolytic anemias results in major metabolic abnormalities: two common metabolite abnormalities include increased levels of 2,3-diphosphoglycerate (2,3-DPG) and decreased levels of adenosine triphosphate (ATP). To better monitor the concentration changes of these metabolites, the development of a reliable LC-MS/MS method to quantitatively profile the concentrations of 2, 3-DPG and ATP in whole blood is essential to understand the effects of investigational therapeutics. Accurate quantification of both compounds imposes great challenges to bioanalytical scientists due to their polar, ionic and endogenous nature. Here we present an LC-MS/MS method for the reliable quantification of 2,3-DPG and ATP from K₂EDTA human whole blood (WB) simultaneously. Whole blood samples were spiked with stable isotope labeled internal standards, processed by protein precipitation extraction, and analyzed using zwitterionic ion chromatography-hydrophilic interaction chromatography (ZIC-HILIC) coupled with tandem mass spectrometry. The linear analytical range of the assay was 50-3000μg/mL. The fit-for-purpose method demonstrated excellent accuracy and precision. The overall accuracy was within ±10.5% (%RE) for both analytes and the intra- and inter-assay precision (%CV) were less than 6.7% and 6.2% for both analytes, respectively. ATP and 2,3-DPG were found to be stable in human K₂EDTA blood for at least 8h at 4°C, 96days when stored at -70°C and after three freeze/thaw cycles. The assay has been successfully applied to K₂EDTA human whole blood samples to support clinical studies.

Reduction in morphological sickling of red cells incubated with quinine

Quinine HCl reduced the formation of reversibly sickled cells in vitro over a concentration range of 0.2-5.0 mM. Quinidine and chloroquine had similar effects. Combinations of quinine and cyanate demonstrated synergistic, antisickling properties in vitro.

An analysis of the mechanism by which cetiedil inhibits the Gardos phenomenon

Energy depletion in the human erythrocyte causes a rise in intracellular calcium. This in turn accelerates the transmembrane movement of potassium and chloride, resulting in cell dehydration. This process, known as the Gardos phenomenon, is inhibited by cetiedil. The present study examines the mechanism by which cetiedil inhibits the Gardos phenomenon. The ability of cetiedil to retard the initial step in the Gardos phenomenon, a rise in intracellular calcium, was first tested. Cetiedil did not prevent calcium accumulation. Cetiedil's ability to inhibit anion movement was next evaluated, as cetiedil could appear to be blocking K movement when in fact it was preventing the movement of its accompanying anion. No inhibitory effect on anion movement was seen. Since cetiedil prevented neither calcium accumulation nor anion movement, it must inhibit the Gardos phenomenon by preventing the opening of the K-specific gate in the erythrocyte membrane. The fact that cetiedil's effect on the Gardos phenomenon could not be removed with repeated cell washing indicates that this effect is irreversible.

A further characterization of the selective K movements observed in human red blood cells following acetylphenylhydrazine exposure

Following brief exposure to acetylphenylhydrazine, the potassium permeability of the human erythrocyte membrane is selectively augmented. While a similar increase in potassium permeability results from the intracellular accumulation of calcium (the Gardos phenomenon), we have found a number of features that allow these two pathways to be distinguished from one another. The acetylphenylhydrazine pathway does not require calcium for its activation, and can be seen even in the presence of a molar excess of the calcium chelator EGTA. The transmembrane potassium movement via this channel has a specific requirement for the anion chloride, and it can be inhibited by furosemide. The potassium that moves through the Gardos pathway, on the other hand, can be accompanied by any permeant anion, and is inhibitable by quinidine or cetiedil. Thus, acetylphenylhydrazine exposure seems to promote K + Cl cotransport, whereas the Gardos pathway represents a potassium conductive channel. While full demonstration of both these pathways requires harsh in vitro manipulation, the large electrochemical potassium gradient favoring the movement of this cation out from the erythrocyte suggests that even a partial activation of either pathway could cause intracellular dehydration and thus contribute importantly to the pathophysiology of in vivo red cell destruction.

Hereditary disorders of red cell enzymes in the Embden-Meyerhof pathway

Recent advances about hereditary disorders of red cell enzymes in the Embden-Meyerhof glycolytic pathway and Rapoport-Leubering cycle are discussed with a stress on pyruvate kinase deficiency, because it is the most common and most intensively studied disorder among them. Broad genetic heterogeneity exists in all the known erythroenzymopathies. Recently, the primary structure of normal human red cell phosphoglycerate kinase has been determined and single amino acid substitutions of four mutant phosphoglycerate kinases have been clarified by Yoshida et al. These studies allowed analysis of structure-function relationships at the molecular level to be carried out more precisely than was previously possible. It is the consensus of the investigators working in this field that the pathogenesis in three-quarters of the congenital nonspherocytic hemolytic anemia patients remains unknown even after adequate red cell enzyme studies and isopropanol test for unstable hemoglobin have been done. This simply means that much studies remain to be worked out in this field.