Kinetic assay shows that increasing red cell volume could be a treatment for sickle cell disease

In vivo measurement of RBC survival in patients with sickle cell disease before or after hematopoietic stem cell transplantation

ABSTRACT

Stable, mixed-donor-recipient chimerism after allogeneic hematopoietic stem cell transplantation (HSCT) for patients with sickle cell disease (SCD) is sufficient for phenotypic disease reversal, and results from differences in donor/recipient-red blood cell (RBC) survival. Understanding variability and predictors of RBC survival among patients with SCD before and after HSCT is critical for gene therapy research which seeks to generate sufficient corrected hemoglobin to reduce polymerization thereby overcoming the red cell pathology of SCD. This study used biotin labeling of RBCs to determine the lifespan of RBCs in patients with SCD compared with patients who have successfully undergone curative HSCT, participants with sickle cell trait (HbAS), and healthy (HbAA) donors. Twenty participants were included in the analysis (SCD pre-HSCT: N = 6, SCD post-HSCT: N = 5, HbAS: N = 6, and HbAA: N = 3). The average RBC lifespan was significantly shorter for participants with SCD pre-HSCT (64.1 days; range, 35-91) compared with those with SCD post-HSCT (113.4 days; range, 105-119), HbAS (126.0 days; range, 119-147), and HbAA (123.7 days; range, 91-147) (P<.001). RBC lifespan correlated with various hematologic parameters and strongly correlated with the average final fraction of sickled RBCs after deoxygenation (P<.001). No adverse events were attributable to the use of biotin and related procedures. Biotin labeling of RBCs is a safe and feasible methodology to evaluate RBC survival in patients with SCD before and after HSCT. Understanding differences in RBC survival may ultimately guide gene therapy protocols to determine hemoglobin composition required to reverse the SCD phenotype as it relates directly to RBC survival. This trial was registered at www.clinicaltrials.gov as #NCT04476277.

Recent developments in the use of pyruvate kinase activators as a new approach for treating sickle cell disease

ABSTRACT

Pyruvate kinase (PK) is a key enzyme in glycolysis, the sole source of adenosine triphosphate, which is essential for all energy-dependent activities of red blood cells. Activating PK shows great potential for treating a broad range of hemolytic anemias beyond PK deficiency, because they also enhance activity of wild-type PK. Motivated by observations of sickle-cell complications in sickle-trait individuals with concomitant PK deficiency, activating endogenous PK offers a novel and promising approach for treating patients with sickle-cell disease.

Drug discovery by a basic research scientist

I was fortunate to do my military service during the Vietnam era as a medical officer at the National Institutes of Health (NIH) in Bethesda, Maryland. My first research at NIH was concerned with making a variety of optical measurements on nucleic acid bases and proteins, including single crystal spectra in linearly polarized light and near infrared circular dichroism, interpreting the spectra using molecular orbital and crystal field theories. What I do now is drug discovery, a field at the opposite end of the scientific spectrum. This article gives a brief account of my transition from spectroscopy to sickle cell hemoglobin polymerization to protein folding to drug discovery for treating sickle cell disease. My lab recently developed a high throughput assay to screen the 12,657 compounds of the California Institute of Biomedical Research ReFrame drug repurposing library. This is a precious library because the compounds have either been FDA approved or have been tested in clinical trials. Since the 1970s numerous agents have been reported in the literature to inhibit HbS polymerization and/or sickling with only one successful drug, hydroxyurea, and another of dubious value, voxelotor, even though it has been approved by the FDA. Our screen has discovered 106 anti-sickling agents in the ReFrame compound library. We estimate that as many as 21 of these compounds could become oral drugs for treating sickle cell disease because they inhibit at concentrations typical of the free concentrations of oral drugs in human serum.

Phenotypic screening of the ReFRAME drug repurposing library to discover new drugs for treating sickle cell disease

Stem cell transplantation and genetic therapies offer potential cures for patients with sickle cell disease (SCD), but these options require advanced medical facilities and are expensive. Consequently, these treatments will not be available for many years to the majority of patients suffering from this disease. What is urgently needed now is an inexpensive oral drug in addition to hydroxyurea, the only drug approved by the FDA that inhibits sickle-hemoglobin polymerization. Here, we report the results of the first phase of our phenotypic screen of the 12,657 compounds of the Scripps ReFRAME drug repurposing library using a recently developed high-throughput assay to measure sickling times following deoxygenation to 0% oxygen of red cells from sickle trait individuals. The ReFRAME library is a very important collection because the compounds are either FDA-approved drugs or have been tested in clinical trials. From dose-response measurements, 106 of the 12,657 compounds exhibit statistically significant antisickling at concentrations ranging from 31 nM to 10 μM. Compounds that inhibit sickling of trait cells are also effective with SCD cells. As many as 21 of the 106 antisickling compounds emerge as potential drugs. This estimate is based on a comparison of inhibitory concentrations with free concentrations of oral drugs in human serum. Moreover, the expected therapeutic potential for each level of inhibition can be predicted from measurements of sickling times for cells from individuals with sickle syndromes of varying severity. Our results should motivate others to develop one or more of these 106 compounds into drugs for treating SCD.

Fluorescence Lifetime Measurement of Prefibrillar Sickle Hemoglobin Oligomers as a Platform for Drug Discovery in Sickle Cell Disease

The molecular origin of sickle cell disease (SCD) has been known since 1949, but treatments remain limited. We present the first high-throughput screening (HTS) platform for discovering small molecules that directly inhibit sickle hemoglobin (HbS) oligomerization and improve blood flow, potentially overcoming a long-standing bottleneck in SCD drug discovery. We show that at concentrations far below the threshold for nucleation and rapid polymerization, deoxygenated HbS forms small assemblies of multiple α2β2 tetramers. Our HTS platform leverages high-sensitivity fluorescence lifetime measurements that monitor these temporally stable prefibrillar HbS oligomers. We show that this approach is sensitive to compounds that inhibit HbS polymerization with or without modulating hemoglobin oxygen binding affinity. We also report the results of a pilot small-molecule screen in which we discovered and validated several novel inhibitors of HbS oligomerization.

The erythroid K-Cl cotransport inhibitor [(dihydroindenyl)oxy]acetic acid blocks erythroid Ca2+-activated K+ channel KCNN4

Red cell volume is a major determinant of HbS concentration in sickle cell disease. Cellular deoxy-HbS concentration determines the delay time, the interval between HbS deoxygenation and deoxy-HbS polymerization. Major membrane transporter protein determinants of sickle red cell volume include the SLC12/KCC K-Cl cotransporters KCC3/SLC12A6 and KCC1/SLC12A4, and the KCNN4/KCa3.1 Ca2+-activated K+ channel (Gardos channel). Among standard inhibitors of KCC-mediated K-Cl cotransport, only [(dihydroindenyl)oxy]acetic acid (DIOA) has been reported to lack inhibitory activity against the related bumetanide-sensitive erythroid Na-K-2Cl cotransporter NKCC1/SLC12A2. DIOA has been often used to inhibit K-Cl cotransport when studying the expression and regulation of other K+ transporters and K+ channels. We report here that DIOA at concentrations routinely used to inhibit K-Cl cotransport can also abrogate activity of the KCNN4/KCa3.1 Gardos channel in human and mouse red cells and in human sickle red cells. DIOA inhibition of A23187-stimulated erythroid K+ uptake (Gardos channel activity) was chloride-independent and persisted in mouse red cells genetically devoid of the principal K-Cl cotransporters KCC3 and KCC1. DIOA also inhibited YODA1-stimulated, chloride-independent erythroid K+ uptake. In contrast, DIOA exhibited no inhibitory effect on K+ influx into A23187-treated red cells of Kcnn4-/- mice. DIOA inhibition of human KCa3.1 was validated (IC50 42 µM) by whole cell patch clamp in HEK-293 cells. RosettaLigand docking experiments identified a potential binding site for DIOA in the fenestration region of human KCa3.1. We conclude that DIOA at concentrations routinely used to inhibit K-Cl cotransport can also block the KCNN4/KCa3.1 Gardos channel in normal and sickle red cells.

Temperature-dependent Self assembly of biofilaments during red blood cell sickling

Molecular self-assembly plays vital role in various biological functions. However, when aberrant molecules self-assemble to form large aggregates, it can give rise to various diseases. For example, the sickle cell disease andAlzheimer's disease are caused by self-assembled hemoglobin fibers and amyloid plaques, respectively. Here we studythe assembly kinetics of such fibers using kinetic Monte- Carlo simulation. We focus on the initial lag time of thesehighly stochastic processes, during which self-assembly is very slow. The lag time distributions turn out to be similarfor two very different regimes of polymerization, namely, a) when polymerization is slow and depolymerization is fast,and b) the opposite case, when polymerization is fast and depolymerization is slow. Using temperature dependent on-and off-rates for hemoglobin fiber growth, reported in recent in-vitro experiments, we show that the mean lag time canexhibit non-monotonic behavior with respect to change in temperature.

High-Throughput Assay to Screen Small Molecules for Their Ability to Prevent Sickling of Red Blood Cells

Sickle cell disease (SCD) is an inherited disorder of hemoglobin (Hb); approximately 300,000 babies are born worldwide with SCD each year. In SCD, fibers of polymerized sickle Hb (HbS) form in red blood cells (RBCs), which cause RBCs to develop their characteristic "sickled" shape, resulting in hemolytic anemia and numerous vascular complications including vaso-occlusive crises. The development of novel antisickling compounds will provide new therapeutic options for patients with SCD. We developed a high-throughput "sickling assay" that is based on an automated high-content imaging system to quantify the effects of hypoxia on the shape and size of RBCs from HbSS SCD patients (SS RBCs). We used this assay to screen thousands of compounds for their ability to inhibit sickling. In the assay, voxelotor (an FDA-approved medication used to treat SCD) prevented sickling with a z'-factor > 0.4, suggesting that the assay is capable of identifying compounds that inhibit sickling. We screened the Broad Repurposing Library of 5393 compounds for their ability to prevent sickling in 4% oxygen/96% nitrogen. We identified two compounds, SNS-314 mesylate and voxelotor itself, that successfully prevented sickling. SNS-314 mesylate prevented sickling in the absence of oxygen, while voxelotor did not, suggesting that SNS-314 mesylate acts by a mechanism that is different from that of voxelotor. The sickling assay described in this study will permit the identification of additional, novel antisickling compounds, which will potentially expand the therapeutic options for SCD.

GBT1118, a voxelotor analog, protects red blood cells from damage during severe hypoxia

A lack of objective metrics in Sickle Cell Disease (SCD) makes it difficult to assess individual patient therapy options or assess the effects of therapy. This is further complicated by mechanisms of action involving multiple interconnected effects, that combine to relieve SCD symptoms. In 2019, based on the increase in hemoglobin concentration observed in the HOPE trial, the Food and Drug Administration approved voxelotor (Oxbryta®, Global Blood Therapeutics) for SCD patients 12 years and older. The main mechanism of action for voxelotor was increased hemoglobin-oxygen affinity, but other mechanisms may apply. In this study, we assessed the effect of GBT1118, an Oxbryta analog, on hypoxia-induced lethal and sub-hemolytic red blood cell (RBC) membrane damage using RBC Mechanical Fragility (MF), a metric of existing membrane damage and prospective hemolysis. RBC MF was measured non-invasively using a proprietary system comprising an electromagnetic bead mill and fiberoptic spectrophotometry detection. Three cycles of severe hypoxia (<5% oxygenated hemoglobin) with follow-up reoxygenation resulted in a significant increase in RBC MF for all SCD (Hb-S >60%) samples. Supplementation with GBT1118 caused no significant changes in pre-hypoxia RBC MF. However, following GBT1118 treatment, cell stability showed significantly less degradation, as evidenced by a significantly smaller RBC MF increase after three cycles of hypoxia-reoxygenation. These findings indicate that GBT1118 prevents hypoxia-induced membrane damage in sickled RBC, in part by alternative mechanisms not associated with induced changes in hemoglobin-oxygen affinity.

Impact of hemoglobin biophysical studies on molecular pathogenesis and drug therapy for sickle cell disease

Basic research on hemoglobin has been essential for understanding the origin and treatment of many hematological disorders due to abnormal hemoglobins. The most important of the hemoglobinopathies is sickle cell disease - Linus Pauling's "molecular disease" that gave birth to molecular medicine. In this review, I will describe the contributions of basic biophysical research on normal and sickle cell hemoglobin (HbS) to understanding the molecular pathogenesis of the disease and providing the conceptual basis for the various approaches to drug therapy that target HbS polymerization. Most prominent among these are the experimental results on the solubility of HbS as a function of oxygen saturation explained by the allosteric model of Monod, Wyman, and Changeux and the Gill-Wyman thermodynamic linkage relation between solubility and oxygen binding, the solubility of mixtures of HbS with normal or fetal hemoglobin explained by Minton's thermodynamic model, and the highly unusual kinetics of HbS polymerization explained by a novel double nucleation mechanism that also accounts for the aggregation kinetics of the Alzheimer's peptide. The HbS polymerization kinetics are of great importance to understanding the pathophysiology and clinical course, as well as guiding drug development for treating this common and severe disease. The article focuses primarily on experimental and theoretical results from my lab, so it is not a comprehensive review of the subject.

Treatment of sickle cell disease by increasing oxygen affinity of hemoglobin

The issue of treating sickle cell disease with drugs that increase hemoglobin oxygen affinity has come to the fore with the FDA approval in 2019 of voxelotor, the only anti-sickling drug approved since hydroxyurea in 1998. Voxelotor reduces sickling by increasing the concentration of the non-polymerizing, high oxygen affinity R (oxy) conformation of HbS. Treatment of sickle cell patients with voxelotor increases Hb levels and decreases indicators of hemolysis, but with no indication as yet that it reduces the frequency of pain episodes. Here we use the allosteric model of Monod, Wyman, and Changeux to simulate whole blood oxygen dissociation curves and red cell sickling in the absence and presence of voxelotor under the in vivo conditions of rapid oxygen pressure decreases. Our modeling agrees with experiments using a new robust assay, which shows the very large, expected decrease in sickling from the drug. The modeling indicates, however, that the increase in oxygen delivery from reduced sickling is largely offset by the increase in oxygen affinity. The net result is that the drug increases overall oxygen delivery only at the very lowest oxygen pressures. Reduction of sickling does, however, mitigate against red cell damage and explains the observed decrease in hemolysis. More importantly, our modeling of in vivo oxygen dissociation, sickling, and oxygen delivery suggests that drugs that increase fetal hemoglobin or decrease MCHC, should be more therapeutically effective than drugs that increase oxygen affinity.

Advances in Sickle Cell Disease Treatments

Sickle Cell Disease (SCD) is an inherited disorder of red blood cells that is caused by a single mutation in the β -globin gene. The disease, which afflicts millions of patients worldwide mainly in low income countries, is characterized by high morbidity, mortality and low life expectancy. The new pharmacological and non-pharmacological strategies for SCD is urgent in order to promote treatments able to reduce patient's suffering and improve their quality of life. Since the FDA approval of HU in 1998, there have been few advances in discovering new drugs; however, in the last three years voxelotor, crizanlizumab, and glutamine have been approved as new therapeutic alternatives. In addition, new promising compounds have been described to treat the main SCD symptoms. Herein, focusing on drug discovery, we discuss new strategies to treat SCD that have been carried out in the last ten years to discover new, safe, and effective treatments. Moreover, non-pharmacological approaches, including red blood cell exchange, gene therapy and hematopoietic stem cell transplantation will be presented.

MWC allosteric model explains unusual hemoglobin-oxygen binding curves from sickle cell drug binding

An oxygen-affinity-modifying drug, voxelotor, has very recently been approved by the FDA for treatment of sickle cell disease. The proposed mechanism of action is by preferential binding of the drug to the R quaternary conformation, which cannot copolymerize with the T conformation to form sickle fibers. Here, we report widely different oxygen dissociation and oxygen association curves for normal blood in the presence of voxelotor and interpret the results in terms of the allosteric model of Monod, Wyman, and Changeux with the addition of drug binding. The model does remarkably well in quantitatively explaining a complex data set with just the addition of drug binding and dissociation rates for the R and T conformations. Whereas slow dissociation of the drug from R results in time-independent dissociation curves, the changing association curves result from slow dissociation of the drug from T, as well as extremely slow binding of the drug to T. By calculating true equilibrium curves from the model parameters, we show that there would be a smaller decrease in oxygen delivery from the left shift in the dissociation curve caused by drug binding if drug binding and dissociation for both R and T were rapid. Our application of the Monod, Wyman, and Changeux model demonstrates once more its enormous power in explaining many different kinds of experimental results for hemoglobin. It should also be helpful in analyzing oxygen binding and in vivo delivery in future investigations of oxygen-affinity-modifying drugs for sickle cell disease.

Emerging drugs in randomized controlled trials for sickle cell disease: are we on the brink of a new era in research and treatment?

Introduction: Sickle cell disease (SCD) is caused by a mutation in the HBB gene which is key for making a component of hemoglobin. The mutation leads to the formation of an abnormal hemoglobin molecule called sickle hemoglobin (HbS). SCD is a chronic, complex disease with a multiplicity of pathophysiological targets; it has high morbidity and mortality.Hydroxyurea has for many years been the only approved drug for SCD; hence, the development of new therapeutics is critical.Areas covered: This article offers an overview of the key studies of new therapeutic options for SCD. We searched the PubMed database and Cochrane Database of Systemic Reviews for agents in early phase clinic trials and preclinical development.Expert opinion: Although knowledge of SCD has progressed, patient survival and quality of life must be improved. Phase II and phase III clinical trials investigating pathophysiology-based novel agents show promising results in the clinical management of SCD acute events. The design of long-term clinical studies is necessary to fully understand the clinical impact of these new therapeutics on the natural history of the disease. Furthermore, the building of global collaborations will enhance the clinical management of SCD and the design of primary outcomes of future clinical trials.

Rational Drug Design of Peptide-Based Therapies for Sickle Cell Disease

Sickle cell disease (SCD) is a group of inherited disorders affecting red blood cells, which is caused by a single mutation that results in substitution of the amino acid valine for glutamic acid in the sixth position of the β-globin chain of hemoglobin. These mutant hemoglobin molecules, called hemoglobin S, can polymerize upon deoxygenation, causing erythrocytes to adopt a sickled form and to suffer hemolysis and vaso-occlusion. Until recently, only two drug therapies for SCD, which do not even fully address the manifestations of SCD, were approved by the United States (US) Food and Drug Administration. A third treatment was newly approved, while a monoclonal antibody preventing vaso-occlusive crises is also now available. The complex nature of SCD manifestations provides multiple critical points where drug discovery efforts can be and have been directed. These notwithstanding, the need for new therapeutic approaches remains high and one of the recent efforts includes developments aimed at inhibiting the polymerization of hemoglobin S. This review focuses on anti-sickling approaches using peptide-based inhibitors, ranging from individual amino acid dipeptides investigated 30–40 years ago up to more promising 12- and 15-mers under consideration in recent years.

High-throughput assessment of hemoglobin polymer in single red blood cells from sickle cell patients under controlled oxygen tension

Sickle cell disease (SCD) is caused by a variant hemoglobin molecule that polymerizes inside red blood cells (RBCs) in reduced oxygen tension. Treatment development has been slow for this typically severe disease, but there is current optimism for curative gene transfer strategies to induce expression of fetal hemoglobin or other nonsickling hemoglobin isoforms. All SCD morbidity and mortality arise directly or indirectly from polymer formation in individual RBCs. Identifying patients at highest risk of complications and treatment candidates with the greatest curative potential therefore requires determining the amount of polymer in individual RBCs under controlled oxygen. Here, we report a semiquantitative measurement of hemoglobin polymer in single RBCs as a function of oxygen. The method takes advantage of the reduced oxygen affinity of hemoglobin polymer to infer polymer content for thousands of RBCs from their overall oxygen saturation. The method enables approaches for SCD treatment development and precision medicine.

Quantitative prediction of erythrocyte sickling for the development of advanced sickle cell therapies

Sickle cell disease is induced by a mutation that converts normal adult hemoglobin to sickle hemoglobin (HbS) and engenders intracellular polymerization of deoxy-HbS and erythrocyte sickling. Development of anti-sickling therapies requires quantitative understanding of HbS polymerization kinetics under organ-specific conditions, which are difficult to assess with existing experimental techniques. Thus, we developed a kinetic model based on the classical nucleation theory to examine the effectiveness of potential anti-sickling drug candidates. We validated this model by comparing its predictability against prior in vivo and in vitro experimental results. We used the model to quantify the efficacy of sickling inhibitors and obtain results consistent with recent screening assays. Global sensitivity analysis on the kinetic parameters in the model revealed that the solubility, nucleation rate prefactor, and oxygen affinity are quantities that dictate HbS polymerization. This finding provides quantitative guidelines for the discovery of intracellular processes to be targeted by sickling inhibitors.

Electrical Impedance Characterization of Erythrocyte Response to Cyclic Hypoxia in Sickle Cell Disease

Cell sickling is the process in which intracellular polymerization of deoxygenated sickle hemoglobin (HbS) leads to distorted, rigid cells, resulting in abnormal blood rheology and painful vaso-occlusion. Current methods for detection of this process mainly rely on optical microscopy of cellular morphology and measurements of cell deformability and blood rheology. As electrical impedance of cells is a sensitive indicator of changes in cellular structure and biophysical characteristics, it can be a promising marker for characterization of abnormal blood rheology and a means more convenient than optics to be integrated into point-of-care devices. In this work, a microfluidics-based electrical impedance sensor has been developed for characterizing the dynamic cell sickling-unsickling processes in sickle blood. The sensor is capable of measuring the continuous variation in the sickle cell suspension due to cyclic hypoxia-induced intracellular HbS polymerization and depolymerization. Simultaneous microscopic imaging of cell morphological change shows the reliability and repeatability of the electrical impedance-based measurements of cell sickling and unsickling processes. Strong correlation is found between the electrical impedance measurement and patients' hematological parameters such as levels of HbS and fetal hemoglobin. The combination of electrical impedance measurement and on-chip hypoxia control provides a promising method for rapid assessment of the dynamic processes of cell sickling and unsickling in patients with sickle cell disease.

"Sickling" in vertebrates: Animal studies vs. sickle cell disease

Before the description of sickled cells in humans, erythrocytes of normal deer were found to deform, or sickle in vitro. Sickling required oxygenation and alkalization; human erythrocytes sickle because their abnormal sickle hemoglobin (HbS) polymerizes following deoxygenation in vivo and in vitro. HbS and some deer hemoglobins polymerize because of specific amino acid contacts between hemoglobin beta-globin chains, although different amino acid residues form the contact points of the polymer. Hemoglobin precipitation is found in other vertebrates. Most often it is a benign in vitro phenomenon. Natural selection established the balanced polymorphism accounting for the high prevalence of HbS where malaria is endemic. A similar selective advantage for "sickling" animal hemoglobins is unproven. The mean corpuscular hemoglobin concentration is about 30 to 35 g/dL. Perhaps during in vitro studies of susceptible animal hemoglobins, minor changes in buffer pH, osmolality and the ligand state of the molecule increases mean cell hemoglobin concentration beyond its solubility limit allowing precipitation.

Interactions of an Anti-Sickling Drug with Hemoglobin in Red Blood Cells from a Patient with Sickle Cell Anemia

The pathophysiology associated with sickle cell disease (SCD) includes hemolytic anemia, vaso-occlusive events, and ultimately end organ damage set off by the polymerization of deoxygenated hemoglobin S (HbS) into long fibers and sickling of red blood cells (RBCs). One approach toward mitigating HbS polymerization is to pharmacologically stabilize the oxygenated (R) conformation of HbS and thereby reduce sickling frequency and SCD pathology. GBT440 is an α-subunit-specific modifying agent that has recently been reported to increase HbS oxygen binding affinity and consequently delay in vitro polymerization. In addition, animal model studies have demonstrated the potential for GBT440 to be a suitable therapeutic for daily oral dosing in humans. Here, we report an optimized method for detecting GBT440 intermediates in human patient hemolysate using a combination of HPLC and mass spectrometry analysis. First, oxygen dissociation curves (ODCs) analyzed from patient blood showed that oxygen affinity increased in a dose dependent manner. Second, HPLC and integrated mass spectrometric analysis collectively confirmed that GBT440 labeling was specific to the α N-terminus thereby ruling out other potential ligand binding sites. Finally, the results from this optimized analytical approach allowed us to detect a stable α-specific GBT440 adduct in the patient's hemolysate in a dose dependent manner. The results and methods presented in this report could therefore potentially help therapeutic monitoring of GBT440 induced oxygen affinity and reveal critical insight into the biophysical properties of GBT440 Hb complexes.

Insight into the complex pathophysiology of sickle cell anaemia and possible treatment

Sickle cell anaemia (SCA) is the consequence of abnormal haemoglobin production due to an inherited point mutation in the β-globin gene. The resulting haemoglobin tetramer is poorly soluble when deoxygenated, and when this is prolonged, intracellular gelation of sickle haemoglobin occurs, followed by haemoglobin polymerisation. If many cycles of sickling and unsickling occur, the red cell membrane will be disrupted leading to haemolysis and vaso-occlusive events. Recent studies have also shown that leucocyte adhesion molecules and nitric oxide (NO) depletion are involved in endothelial damage. New insights in SCA pathophysiology and vascular biology have shown that cell-derived microparticle (MP) generation is also involved in the vaso-occlusion. Endothelial damage is perpetuated by impaired production or increased consumption of protective modulators such as protein C, protein S and NO. New therapeutic interventions should address these aspects of SCA pathogenesis. To date, the only US-FDA-approved therapy to prevent painful vaso-occulsive episodes is hydroxyurea that reduces haemoglobin polymerisation in sickle cells by increasing the production of foetal haemoglobin and L-glutamine. However, several new drugs have been tested in the last years in randomised clinical trials. We here report an update on the current status of knowledge on SCA.

New Therapeutic Options for the Treatment of Sickle Cell Disease

Sickle cell disease (SCD; ORPHA232; OMIM # 603903) is a chronic and invalidating disorder distributed worldwide, with high morbidity and mortality. Given the disease complexity and the multiplicity of pathophysiological targets, development of new therapeutic options is critical, despite the positive effects of hydroxyurea (HU), for many years the only approved drug for SCD. New therapeutic strategies might be divided into (1) pathophysiology-related novel therapies and (2) innovations in curative therapeutic options such as hematopoietic stem cell transplantation and gene therapy. The pathophysiology related novel therapies are: a) Agents which reduce sickling or prevent sickle red cell dehydration; b) Agents targeting SCD vasculopathy and sickle cell-endothelial adhesive events; c) Anti-oxidant agents. This review highlights new therapeutic strategies in SCD and discusses future developments, research implications, and possible innovative clinical trials.

GBT440 improves red blood cell deformability and reduces viscosity of sickle cell blood under deoxygenated conditions

BACKGROUND

In sickle cell disease (SCD), polymerization of hemoglobin S (HbS) leads to the formation of rigid, non-deformable sickled RBCs. Loss of RBC deformability, sickling and irreversible membrane damage causes abnormal blood rheology, and increases viscosity which contributes to vasoocclusion and other SCD pathophysiology. GBT440 (generic name voxelotor) is a novel anti-polymerization and anti-sickling agent currently undergoing clinical evaluation for the treatment of SCD.

OBJECTIVE

The purpose of this study was to determine the effects of GBT440 on deformability of sickle RBCs (SS RBCs) and the hyperviscosity of sickle cell blood (SS blood).

METHODS

The mechanical and rheological properties of GBT440-treated SS RBCs were measured using micropipette and filtration techniques. The viscosity of sickle blood was measured using a Wells-Brookfield cone/plate viscometer.

RESULTS

GBT440 restored movement of deoxygenated SS RBCs through a gel filtration column and reduced the pressure required to pass SS RBCs through a polycarbonate filter. Moreover, GBT440 decreased the membrane shear elastic modulus of SS RBCs assessed via micropipette aspiration and reduced the hyperviscosity of SS blood under deoxygenated conditions.

CONCLUSIONS

GBT440 maintains SS RBC deformability and improves SS blood viscosity by inhibiting HbS polymerization under deoxygenated conditions. These results further support development of GBT440 as a disease-modifying agent in SCD patients.

Cell Volume Controls Protein Stability and Compactness of the Unfolded State

Macromolecular crowding is widely accepted as one of the factors that can alter protein stability, structure, and function inside cells. Less often considered is that crowding can be dynamic: as cell volume changes, either as a result of external duress or in the course of the cell cycle, water moves in or out through membrane channels, and crowding changes in tune. Both theory and in vitro experiments predict that protein stability will be altered as a result of crowding changes. However, it is unclear how much the structural ensemble is altered as crowding changes in the cell. To test this, we look at the response of a FRET-labeled kinase to osmotically induced volume changes in live cells. We examine both the folded and unfolded states of the kinase by changing the temperature of the media surrounding the cell. Our data reveals that crowding compacts the structure of its unfolded ensemble but stabilizes the folded protein. We propose that the structure of proteins lacking a rigid, well-defined tertiary structure could be highly sensitive to both increases and decreases in cell volume. Our findings present a possible mechanism for disordered proteins to act as sensors and actuators of cell cycle or external stress events that coincide with a change in macromolecular crowding.

Oxygen-dependent flow of sickle trait blood as an in vitro therapeutic benchmark for sickle cell disease treatments

Although homozygous sickle cell disease is often clinically severe, the corresponding heterozygous state, sickle cell trait, is almost completely benign despite the fact that there is only a modest difference in sickle hemoglobin levels between the two conditions. In both conditions, hypoxia can lead to polymerization of sickle hemoglobin, changes in red cell mechanical properties, and impaired blood flow. Here, we test the hypothesis that differences in the oxygen-dependent rheological properties in the two conditions might help explain the difference in clinical phenotypes. We use a microfluidic platform that permits quantification of blood rheology under defined oxygen conditions in physiologically sized microchannels and under physiologic shear rates. We find that, even with its lower sickle hemoglobin concentration, sickle trait blood apparent viscosity increases with decreasing oxygen tension and may stop flowing under completely anoxic conditions, though far less readily than the homozygous condition. Sickle cell trait blood flow becomes impaired at significantly lower oxygen tension than sickle cell disease. We also demonstrate how sickle cell trait can serve as a benchmark for sickle cell disease therapies. We characterize the rheological effects of exchange transfusion therapy by mixing sickle blood with nonsickle blood and quantifying the transfusion targets for sickle hemoglobin composition below which the rheological response resembles sickle trait. These studies quantify the differences in blood flow phenotypes of sickle cell disease and sickle cell trait, and they provide a potentially powerful new benchmark for evaluating putative therapies in vitro.

Theoretical Simulation of Red Cell Sickling Upon Deoxygenation Based on the Physical Chemistry of Sickle Hemoglobin Fiber Formation

The polymerization of the mutant hemoglobin S upon deoxygenation to form fibers in red blood cells of patients suffering from sickle-cell anemia results in changes in cell shape and rigidity, also known as sickling, which underlie the pathology of the disease. While much has been learned about the fundamental physical chemistry of the polymerization process, transferring these insights to sickling of red cells under in vivo conditions requires being able to monitor, and ultimately predict, the time course of cellular sickling under physiological conditions of deoxygenation. To this end, we have developed an experimental technique for tracking the temporal evolution of the sickling of red blood cells under laboratory deoxygenation conditions, based on the automated analysis of sequences of microscope images and machine-learning analysis to characterize cell morphology. As an aid in the quantitative understanding of these experiments, we have developed a computational framework for simulating the time dependence of sickling in populations of red blood cells which incorporates the current theoretical and empirical understanding of the physical chemistry of the sickling process. In order to apply these techniques to our experiments, we have theoretically determined the time course of deoxygenation by solving the diffusion equation for oxygen in our experimental geometry. With this combined description, we are able to reproduce our experimentally observed kinetics of sickling, suggesting that our theoretical approach should be applicable to physiological deoxygenation scenarios.

Inflammation in sickle cell disease

The primary β-globin gene mutation that causes sickle cell disease (SCD) has significant pathophysiological consequences that result in hemolytic events and the induction of the inflammatory processes that ultimately lead to vaso-occlusion. In addition to their role in the initiation of the acute painful vaso-occlusive episodes that are characteristic of SCD, inflammatory processes are also key components of many of the complications of the disease including autosplenectomy, acute chest syndrome, pulmonary hypertension, leg ulcers, nephropathy and stroke. We, herein, discuss the events that trigger inflammation in the disease, as well as the mechanisms, inflammatory molecules and cells that propagate these inflammatory processes. Given the central role that inflammation plays in SCD pathophysiology, many of the therapeutic approaches currently under pre-clinical and clinical development for the treatment of SCD endeavor to counter aspects or specific molecules of these inflammatory processes and it is possible that, in the future, we will see anti-inflammatory drugs being used either together with, or in place of, hydroxyurea in those SCD patients for whom hematopoietic stem cell transplants and evolving gene therapies are not a viable option.

Targeting HbS Polymerization

The mutation of β6 from glu to val in hemoglobin is responsible for the polymer formation that leads to vaso-occlusion, and a range of severe consequences in sickle cell disease. The treatment of the disease can be addressed in many ways, but the prevention of polymer formation is one of the most fundamental approaches one can take. Such prevention includes affecting the polymer structure, or dilution of the fraction of polymerizable hemoglobin. The latter approach includes (1) induction of HbF, which does not itself, nor in hybrid form, join sickle polymers, or (2) restricting the allosteric change in hemoglobin that occurs in oxygen delivery, and which is required for polymer formation. These approaches will be critically reviewed, as well as the most recent developments that show the benefits of simply swelling the volume of the red cell.

How does solvation in the cell affect protein folding and binding?

The cellular environment is highly diverse and capable of rapid changes in solute composition and concentrations. Decades of protein studies have highlighted their sensitivity to solute environment, yet these studies were rarely performed in situ. Recently, new techniques capable of monitoring proteins in their natural context within a live cell have emerged. A recurring theme of these investigations is the importance of the often-neglected cellular solvation environment to protein function. An emerging consensus is that protein processes in the cell are affected by a combination of steric and non-steric interactions with this solution. Here we explain how protein surface area and volume changes control these two interaction types, and give recent examples that highlight how even mild environmental changes can alter cellular processes.

Potential therapeutic action of nitrite in sickle cell disease

Sickle cell disease is caused by a mutant form of hemoglobin that polymerizes under hypoxic conditions, increasing rigidity, fragility, calcium influx-mediated dehydration, and adhesivity of red blood cells. Increased red cell fragility results in hemolysis, which reduces nitric oxide (NO) bioavailability, and induces platelet activation and inflammation leading to adhesion of circulating blood cells. Nitric Oxide inhibits adhesion and platelet activation. Nitrite has emerged as an attractive therapeutic agent that targets delivery of NO activity to areas of hypoxia through bioactivation by deoxygenated red blood cell hemoglobin. In this study, we demonstrate anti-platelet activity of nitrite at doses achievable through dietary interventions with comparison to similar doses with other NO donating agents. Unlike other NO donating agents, nitrite activity is shown to be potentiated in the presence of red blood cells in hypoxic conditions. We also show that nitrite reduces calcium associated loss of phospholipid asymmetry that is associated with increased red cell adhesion, and that red cell deformability is also improved. We show that nitrite inhibits red cell adhesion in a microfluidic flow-channel assay after endothelial cell activation. In further investigations, we show that leukocyte and platelet adhesion is blunted in nitrite-fed wild type mice compared to control after either lipopolysaccharide- or hemolysis-induced inflammation. Moreover, we demonstrate that nitrite treatment results in a reduction in adhesion of circulating blood cells and reduced red blood cell hemolysis in humanized transgenic sickle cell mice subjected to local hypoxia. These data suggest that nitrite is an effective anti-platelet and anti-adhesion agent that is activated by red blood cells, with enhanced potency under physiological hypoxia and in venous blood that may be useful therapeutically.

Oxidative pathways in the sickle cell and beyond

Polymerization of deoxy sickle cell hemoglobin (HbS) is well recognized as the primary event that triggers the classic cycles of sickling/unsickling of patients red blood cells (RBCs). RBCs are also subjected to continuous endogenous and exogenous oxidative onslaughts resulting in hemolytic rate increases which contribute to the evolution of vasculopathies associated with this disease. Compared to steady-state conditions, the occurrences of vaso-occlusive crises increase the levels of both RBC-derived microparticles as well as extracellular Hb in circulation. Common byproduct resulting from free Hb oxidation and from Hb-laden microparticles is heme (now recognized as damage associated molecular pattern (DAMP) molecule) which has been shown to initiate inflammatory responses. This review provides new insights into the interplay between microparticles, free Hb and heme focusing on Hb's pseudoperoxidative activity that drives RBC's cytosolic, membrane changes as well as oxidative toxicity towards the vascular system. Emerging antioxidative strategies that include the use of protein and heme scavengers in controlling Hb oxidative pathways are discussed.

Genome-wide association study of erythrocyte density in sickle cell disease patients

Deoxy-hemoglobin S polymerization into rigid fibers is the direct cause of the clinical sequelae observed in sickle cell disease (SCD). The rate of polymerization of sickle hemoglobin is determined primarily by intracellular hemoglobin concentration, itself dependent on the amount of sickle hemoglobin and on red blood cell (RBC) volume. Dense, dehydrated RBC (DRBC) are observed in SCD patients, and their number correlates with hemolytic parameters and complications such as renal dysfunction, leg ulcers and priapism. To identify new genes involved in RBC hydration in SCD, we performed the first genome-wide association study for DRBC in 374 sickle cell anemia (HbSS) patients. We did not find genome-wide significant results, indicating that variants that modulate DRBC have modest-to-weak effects. A secondary analysis demonstrated a nominal association (P=0.003) between DRBC in SCD patients and a variant associated with mean corpuscular hemoglobin concentration (MCHC) in non-anemic individuals. This intronic variant controls the expression of ATP2B4, the main calcium pump in erythrocytes. Our study highlights ATP2B4 as a promising target for modulation of RBC hydration in SCD patients.

Treating sickle cell disease by targeting HbS polymerization

Although the root cause of sickle cell disease is the polymerization of hemoglobin S (HbS) to form fibers that make red cells less flexible, most drugs currently being assessed in clinical trials are targeting the downstream sequelae of this primary event. Less attention has been devoted to investigation of the multiple ways in which fiber formation can be inhibited. In this article, we describe the molecular rationale for 5 distinct approaches to inhibiting polymerization and also discuss progress with the few antipolymerization drugs currently in clinical trials.