The LSD1 inhibitor RN-1 recapitulates the fetal pattern of hemoglobin synthesis in baboons (P. anubis)

Red Blood Cells as Therapeutic Target to Treat Sickle Cell Disease

Significance: Sickle cell disease (SCD) is the most common inherited diathesis affecting mostly underserved populations globally. SCD is characterized by chronic pain and fatigue, severe acute painful crises requiring hospitalization and opioids, strokes, multiorgan damage, and a shortened life span. Symptoms may appear shortly after birth, and, in less developed countries, most children with SCD die before attaining age 5. Hematopoietic stem cell transplant and gene therapy offer a curative therapeutic approach, but, due to many challenges, are limited in their availability and effectiveness for a majority of persons with SCD. A critical unmet need is to develop safe and effective novel targeted therapies. A wide array of drugs currently undergoing clinical investigation hold promise for an expanded pharmacological armamentarium against SCD. Recent Advances: Hydroxyurea, the most widely used intervention for SCD management, has improved the survival in the Western world and more recently, voxelotor (R-state-stabilizer), l-glutamine, and crizanlizumab (anti-P-selectin antibody) have been approved by the Food and Drug Administration (FDA) for use in SCD. The recent FDA approval emphasizes the need to revisit the advances in understanding the core pathophysiology of SCD to accelerate novel evidence-based strategies to treat SCD. The biomechanical breakdown of erythrocytesis, the core pathophysiology of SCD, is associated with intrinsic factors, including the composition of hemoglobin, membrane integrity, cellular volume, hydration, andoxidative stress. Critical Issues and Future Directions: In this context, this review focuses on advances in emerging nongenetic interventions directed toward the therapeutic targets intrinsic to sickle red blood cells (RBCs), which can prevent impaired rheology of RBCs to impede disease progression and reduce the sequelae of comorbidities, including pain, vasculopathy, and organ damage. In addition, given the intricate pathophysiology of the disease, it is unlikely that a single pharmacotherapeutic intervention will comprehensively ameliorate the multifaceted complications associated with SCD. However, the availability of multiple drug options affords the opportunity for individualized therapeutic regimens tailored to specific SCD-related complications. Furthermore, it opens avenues for combination drug therapy, capitalizing on distinct mechanisms of action and profiles of adverse effects.

Effect of the LSD1 inhibitor RN-1 on γ-globin and global gene expression during erythroid differentiation in baboons (Papio anubis)

Elevated levels of Fetal Hemoglobin interfere with polymerization of sickle hemoglobin thereby reducing anemia, lessening the severity of symptoms, and increasing life span of patients with sickle cell disease. An affordable, small molecule drug that stimulates HbF expression in vivo would be ideally suited to treat the large numbers of SCD patients that exist worldwide. Our previous work showed that administration of the LSD1 (KDM1A) inhibitor RN-1 to normal baboons increased Fetal Hemoglobin (HbF) and was tolerated over a prolonged treatment period. HbF elevations were associated with changes in epigenetic modifications that included increased levels of H3K4 di-and tri-methyl lysine at the γ-globin promoter. While dramatic effects of the loss of LSD1 on hematopoietic differentiation have been observed in murine LSD1 gene deletion and silencing models, the effect of pharmacological inhibition of LSD1 in vivo on hematopoietic differentiation is unknown. The goal of these experiments was to investigate the in vivo mechanism of action of the LSD1 inhibitor RN-1 by determining its effect on γ-globin expression in highly purified subpopulations of bone marrow erythroid cells enriched for varying stages of erythroid differentiation isolated directly from baboons treated with RN-1 and also by investigating the effect of RN1 on the global transcriptome in a highly purified population of proerythroblasts. Our results show that RN-1 administered to baboons targets an early event during erythroid differentiation responsible for γ-globin repression and increases the expression of a limited number of genes including genes involved in erythroid differentiation such as GATA2, GFi-1B, and LYN.

Combinatorial targeting of epigenome-modifying enzymes with decitabine and RN-1 synergistically increases HbF

Increased fetal hemoglobin (HbF) levels reduce the symptoms of sickle cell disease (SCD) and increase the lifespan of patients. Because curative strategies for bone marrow transplantation and gene therapy technologies remain unavailable to a large number of patients, the development of a safe and effective pharmacological therapy that increases HbF offers the greatest potential for disease intervention. Although hydroxyurea increases HbF, a substantial proportion of patients fail to demonstrate an adequate response. Pharmacological inhibitors of DNA methyltransferase (DNMT1) and lysine-specific demethylase 1A (LSD1), 2 epigenome-modifying enzymes associated with the multiprotein corepressor complex recruited to the repressed γ-globin gene, are powerful in vivo inducers of HbF. The hematological side effects of these inhibitors limit feasible clinical exposures. We evaluated whether administering these drugs in combination could reduce the dose and/or time of exposure to any single agent to minimize adverse effects, while achieving additive or synergistic increases in HbF. The DNMT1 inhibitor decitabine (0.5 mg/kg per day) and the LSD1 inhibitor RN-1 (0.25 mg/kg per day) administered in combination 2 days per week produced synergistic increases in F-cells, F-reticulocytes, and γ-globin messenger RNA in healthy baboons. Large increases in HbF and F-cells were observed in healthy, nonanemic, and anemic (phlebotomized) baboons. Combinatorial therapy targeting epigenome-modifying enzymes could thus be a useful strategy for producing larger increases in HbF to modify the clinical course of SCD.

Reticulocyte mitochondrial retention increases reactive oxygen species and oxygen consumption in mouse models of sickle cell disease and phlebotomy-induced anemia

Sickle cell disease (SCD) is caused by a mutation of the β-globin gene that results in the production of hemoglobin S (HbS). People with SCD experience anemia, severe acute pain episodes, persistent chronic pain, multiorgan damage, and a reduced life span. The pathophysiology of SCD caused by the polymerization of HbS on deoxygenation results in red cell deformability and the generation of reactive oxygen species (ROS). These 2 factors lead to red cell fragility and hemolysis. Reticulocytosis is an independent predictor of disease morbidity and mortality in SCD. We previously established that humans and mice with SCD exhibit abnormal mitochondrial retention in erythrocytes increasing ROS-associated hemolysis. Here, we investigated the hypothesis that mitochondrial retention and increased ROS are a consequence of stress erythropoiesis. Our results show clearly that stress erythropoiesis in phlebotomized, anemic AA mice results in mitochondrial retention and increased ROS in reticulocytes. We observed that elevated mitochondrial retention in reticulocytes also alters oxygen consumption and potentially contributes to increased HbS polymerization and red blood cell hemolysis. Therefore, these events occurring due to stress erythropoiesis contribute significantly to the pathology of SCD and suggest new therapeutic targets.

Epigenetic Regulation of β-Globin Genes and the Potential to Treat Hemoglobinopathies through Epigenome Editing

Beta-like globin gene expression is developmentally regulated during life by transcription factors, chromatin looping and epigenome modifications of the β-globin locus. Epigenome modifications, such as histone methylation/demethylation and acetylation/deacetylation and DNA methylation, are associated with up- or down-regulation of gene expression. The understanding of these mechanisms and their outcome in gene expression has paved the way to the development of new therapeutic strategies for treating various diseases, such as β-hemoglobinopathies. Histone deacetylase and DNA methyl-transferase inhibitors are currently being tested in clinical trials for hemoglobinopathies patients. However, these approaches are often uncertain, non-specific and their global effect poses serious safety concerns. Epigenome editing is a recently developed and promising tool that consists of a DNA recognition domain (zinc finger, transcription activator-like effector or dead clustered regularly interspaced short palindromic repeats Cas9) fused to the catalytic domain of a chromatin-modifying enzyme. It offers a more specific targeting of disease-related genes (e.g., the ability to reactivate the fetal γ-globin genes and improve the hemoglobinopathy phenotype) and it facilitates the development of scarless gene therapy approaches. Here, we summarize the mechanisms of epigenome regulation of the β-globin locus, and we discuss the application of epigenome editing for the treatment of hemoglobinopathies.

The state of the art of fetal hemoglobin-inducing agents

INTRODUCTION

Sickle cell anemia (SCA) is a hematological genetic disorder caused by a mutation in the gene of the β-globin. Pharmacological treatments will continue to be an important approach, including the strategy to induce fetal hemoglobin (HbF).

AREAS COVERED

Here, we analyzed the articles described in the literature regarding the drug discovery of HbF inducers. The main approaches for such strategy will be discussed, highlighting those most promising.

EXPERT OPINION

The comprehension of the mechanisms involved in the β-globin regulation is the main key to design new drugs to induce HbF. Among the strategies, gamma-globin regulation by epigenetic enzymes seems to be a promising approach to be pursued, although the comprehension of the selectivity role for those new drugs is crucial to reduce adverse effects. The low druggability of transcription factors and their vital role in embryonic human development are critical points that should be taken in account for drug design. The guanylate cyclase and the NO/cGMP signaling pathway seem to be promising not only for HbF induction, but also for the protective effects in the cardiovascular system. The association of drugs acting through different mechanisms to induce HbF seems to be promising for the discovery of new drugs.

Lysine-Specific Demethylase 1 (LSD1/KDM1A) Inhibition as a Target for Disease Modification in Myelofibrosis

Myelofibrosis (MF) is the most symptomatic form of myeloproliferative neoplasm and carries the worst outcome. Allogeneic hematopoietic stem cell transplantation is the only therapy with potential for cure at present, but is limited by significant mortality and morbidity. JAK inhibition is the mainstay of treatment for intermediate- and high-risk MF. Ruxolitinib is the most widely used JAK1/2 inhibitor and provides durable effects in controlling symptom burden and spleen volumes. Nevertheless, ruxolitinib may not adequately address the underlying disease biology. Its effects on mutant allele burden, bone marrow fibrosis, and the prevention of leukemic transformation are minimal. Multiple small molecules are being tested in multiple phase 2 and 3 studies as either monotherapy or in combination with JAK2 inhibitors. In this review, the role of LSD1/KDM1A inhibition as a potential disease-modification strategy in patients with myelofibrosis is described and discussed.

In Vitro and In Vivo Studies for the Investigation of γ-Globin Gene Induction by Adhatoda vasica: A Pre-Clinical Study of HbF Inducers for β-Thalassemia

Fetal hemoglobin (HbF) is a potent genetic modifier, and the γ-globin gene induction has proven to be a sustainable therapeutic approach for the management of β-thalassemia. In this study, we have evaluated the HbF induction ability of A. vasica in vitro and in vivo, and the identification of potential therapeutic compounds through a bioassay-guided approach. In vitro benzidine-Hb assay demonstrated strong erythroid differentiation of K562 cells by A. vasica extracts. Subsequently, an in vivo study with an aqueous extract of A. vasica (100 mg/kg) showed significant induction of the γ-globin gene and HbF production. While in the acute study, the hematological and biochemical indices were found to be unaltered at the lower dose of A. vasica. Following the bioassay-guided approach, two isolated compounds, vasicinol (1) and vasicine (2) strongly enhanced HbF levels and showed prominent cellular growth kinetics with ample accumulation of total hemoglobin in K562 cultures. High HbF levels were examined by immunofluorescence and flow cytometry analysis, concomitant with the overexpression in the γ-globin gene level. Compound 1 (0.1 µM) and compound 2 (1 µM) resulted in a greater increase in F-cells (90 and 83%) with marked up (8-fold and 5.1-fold) expression of the γ-globin gene, respectively. Molecular docking studies indicated strong binding affinities of (1) and (2) with HDAC2 and KDM1 protein that predict the possible mechanism of compounds in inhibition of these epigenetic regulators in the γ-globin gene reactivation. Altogether, these observations demonstrated the therapeutic usefulness of A. vasica for fostering HbF production in clinical implications for blood disorders.

Pharmacologic induction of PGC-1α stimulates fetal haemoglobin gene expression

Sickle cell disease (SCD) is a genetic disorder that affects millions around the world. Enhancement of fetal γ-globin levels and fetal haemoglobin (HbF) production in SCD patients leads to diminished severity of many clinical features of the disease. We recently identified the transcriptional co-activator PGC-1α as a new protein involved in the regulation of the globin genes. Here, we report that upregulation of PGC-1α by infection with a lentivirus expressing PGC-1α or by the small-molecule PGC-1α agonist ZLN005 in human primary erythroid progenitor CD34⁺ cells induces both fetal γ-globin mRNA and protein expression as well as the percentage of HbF-positive cell (F cells) without significantly affecting cell proliferation and differentiation. We further found that the combination of ZLN005 and hydroxyurea (hydroxycarbamide) exhibited an additive effect on the expression of γ-globin and the generation of F cells from cultured CD34⁺ cells. In addition, ZLN005 induced robust expression of the murine embryonic βh1-globin gene and to a lesser extent, human γ-globin gene expression in sickle mice. These findings suggest that activation of PGC-1α by ZLN005 might provide a new path for modulating HbF levels with potential therapeutic benefit in β-hemoglobinopathies.

Lysine-specific histone demethylase 1 inhibition enhances robust fetal hemoglobin induction in human β0-thalassemia/hemoglobin E erythroid cells

Induction of fetal hemoglobin (HbF) ameliorates the clinical severity of β-thalassemias. Histone methyltransferase LSD1 enzyme removes methyl groups from the activating chromatin mark histone 3 lysine 4 at silenced genes, including the γ-globin genes. LSD1 inhibitor RN-1 induces HbF levels in cultured human erythroid cells. Here, the HbF-inducing activity of RN-1 was investigated in erythroid progenitor cells derived from β⁰-thalassemia/ hemoglobin E (HbE) patients. The significant and reproducible increases in γ-globin transcript and HbF expression upon RN-1 treatment were demonstrated in erythroid cells with divergent HbF baseline levels, the average of HbF induction was 17.7±0.8%. RN-1 at low concentration did not affect viability and proliferation of erythroid cells, but decreases in cell number were observed in cells treated with RN-1 at high concentration. Delayed terminal erythroid differentiation was revealed in β⁰-thalassemia/HbE erythroid cells treated with RN-1 as similar to other compounds that target LSD1 activity. Downregulation of repressors of γ- globin expression; NCOR1 and SOX6, was observed in RN-1 treatment. These findings provide proof of the concept that LSD1 epigenetic enzyme is a potential therapeutic target for β⁰-thalassemia/HbE patients.

An erythroid-to-myeloid cell fate conversion is elicited by LSD1 inactivation

Histone H3 lysine 4 methylation (H3K4Me) is most often associated with chromatin activation, and removing H3K4 methyl groups has been shown to be coincident with gene repression. H3K4Me demethylase KDM1a/LSD1 is a therapeutic target for multiple diseases, including for the potential treatment of β-globinopathies (sickle cell disease and β-thalassemia), because it is a component of γ-globin repressor complexes, and LSD1 inactivation leads to robust induction of the fetal globin genes. The effects of LSD1 inhibition in definitive erythropoiesis are not well characterized, so we examined the consequences of conditional inactivation of Lsd1 in adult red blood cells using a new Gata1creERT2 bacterial artificial chromosome transgene. Erythroid-specific loss of Lsd1 activity in mice led to a block in erythroid progenitor differentiation and to the expansion of granulocyte-monocyte progenitor-like cells, converting hematopoietic differentiation potential from an erythroid fate to a myeloid fate. The analogous phenotype was also observed in human hematopoietic stem and progenitor cells, coincident with the induction of myeloid transcription factors (eg, PU.1 and CEBPα). Finally, blocking the activity of the transcription factor PU.1 or RUNX1 at the same time as LSD1 inhibition rescued myeloid lineage conversion to an erythroid phenotype. These data show that LSD1 promotes erythropoiesis by repressing myeloid cell fate in adult erythroid progenitors and that inhibition of the myeloid-differentiation pathway reverses the lineage switch induced by LSD1 inactivation.

Thalassemias: From gene to therapy

Thalassemias (α, β, γ, δ, δβ, and εγδβ) are the most common genetic disorders worldwide and constitute a heterogeneous group of hereditary diseases characterized by the deficient synthesis of one or more hemoglobin (Hb) chain(s). This leads to the accumulation of unstable non-thalassemic Hb chains, which precipitate and cause intramedullary destruction of erythroid precursors and premature lysis of red blood cells (RBC) in the peripheral blood. Non-thalassemic Hbs display high oxygen affinity and no cooperativity. Thalassemias result from many different genetic and molecular defects leading to either severe or clinically silent hematologic phenotypes. Thalassemias α and β are particularly diffused in the regions spanning from the Mediterranean basin through the Middle East, Indian subcontinent, Burma, Southeast Asia, Melanesia, and the Pacific Islands, whereas δβ-thalassemia is prevalent in some Mediterranean regions including Italy, Greece, and Turkey. Although in the world thalassemia and malaria areas overlap apparently, the RBC protection against malaria parasites is openly debated. Here, we provide an overview of the historical, geographic, genetic, structural, and molecular pathophysiological aspects of thalassemias. Moreover, attention has been paid to molecular and epigenetic pathways regulating globin gene expression and globin switching. Challenges of conventional standard treatments, including RBC transfusions and iron chelation therapy, splenectomy and hematopoietic stem cell transplantation from normal donors are reported. Finally, the progress made by rapidly evolving fields of gene therapy and gene editing strategies, already in pre-clinical and clinical evaluation, and future challenges as novel curative treatments for thalassemia are discussed.

Exploring epigenetic and microRNA approaches for γ-globin gene regulation

Therapeutic interventions aimed at inducing fetal hemoglobin and reducing the concentration of sickle hemoglobin is an effective approach to ameliorating acute and chronic complications of sickle cell disease, exemplified by the long-term use of hydroxyurea. However, there remains an unmet need for the development of additional safe and effective drugs for single agent or combination therapy for individuals with β-hemoglobinopathies. Regulation of the γ-globin to β-globin switch is achieved by chromatin remodeling at the HBB locus on chromosome 11 and interactions of major DNA binding proteins, such as KLF1 and BCL11A in the proximal promoters of the globin genes. Experimental evidence also supports a role of epigenetic modifications including DNA methylation, histone acetylation/methylation, and microRNA expression in γ-globin gene silencing during development. In this review, we will critically evaluate the role of epigenetic mechanisms in γ-globin gene regulation and discuss data generated in tissue culture, pre-clinical animal models, and clinical trials to support drug development to date. The question remains whether modulation of epigenetic pathways will produce sufficient efficacy and specificity for fetal hemoglobin induction and to what extent targeting these pathways form the basis of prospects for clinical therapy.

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.

The methyltransferase PRMT1 regulates γ-globin translation

Induction of fetal hemoglobin to overcome adult β-globin gene deficiency is an effective therapeutic strategy to ameliorate human β-hemoglobinopathies. Previous work has revealed that fetal γ-globin can be translationally induced via integrated stress signaling, but other studies have indicated that activating stress may eventually suppress γ-globin expression transcriptionally. The mechanism by which γ-globin expression is regulated at the translational level remains largely unknown, limiting our ability to determine whether activating stress is a realistic therapeutic option for these disorders. In this study, we performed a functional CRISPR screen targeting protein arginine methyltransferases (PRMTs) to look for changes in γ-globin expression in K562 cells. We not only discovered that several specific PRMTs may block γ-globin transcription, but also revealed PRMT1 as a unique family member that is able to suppress γ-globin synthesis specifically at the translational level. We further identified that a non-AUG uORF within the 5' untranslated region of γ-globin serves as a barrier for translation, which is bypassed upon PRMT1 deficiency. Finally, we found that this novel mechanism of γ-globin suppression could be pharmacologically targeted by the PRMT1 inhibitor, furamidine dihydrochloride. These data raise new questions regarding methyltransferase function and may offer a new therapeutic direction for β-hemoglobinopathies.

Discovery of DS79932728: A Potent, Orally Available G9a/GLP Inhibitor for Treating β-Thalassemia and Sickle Cell Disease

Therapeutic reactivation of the γ-globin genes for fetal hemoglobin (HbF) production is an attractive strategy for treating β-thalassemia and sickle cell disease. It was reported that genetic knockdown of the histone lysine methyltransferase EHMT2/1 (G9a/GLP) is sufficient to induce HbF production. The aim of the present work was to acquire a G9a/GLP inhibitor that induces HbF production sufficiently. It was revealed that tetrahydroazepine has versatility as a side chain in various skeletons. We ultimately obtained a promising aminoindole derivative (DS79932728), a potent and orally bioavailable G9a/GLP inhibitor that was found to induce γ-globin production in a phlebotomized cynomolgus monkey model. This work could facilitate the development of effective new approaches for treating β-thalassemia and sickle cell disease.

When basic science reaches into rational therapeutic design: from historical to novel leads for the treatment of β-globinopathies

PURPOSE OF REVIEW

β-hemoglobinopathies, such as β-Thalassemias (β-Thal) and sickle cell disease (SCD) are among the most common inherited genetic disorders in humans worldwide. These disorders are characterized by a quantitative (β-Thal) or qualitative (SCD) defects in adult hemoglobin production, leading to anemia, ineffective erythropoiesis and severe secondary complications. Reactivation of the fetal globin genes (γ-globin), making-up fetal hemoglobin (HbF), which are normally silenced in adults, represents a major strategy to ameliorate anemia and disease severity.

RECENT FINDINGS

Following the identification of the first 'switching factors' for the reactivation of fetal globin gene expression more than 10 years ago, a multitude of novel leads have recently been uncovered.

SUMMARY

Recent findings provided invaluable functional insights into the genetic and molecular networks controlling globin genes expression, revealing that complex repression systems evolved in erythroid cells to maintain HbF silencing in adults. This review summarizes these unique and exciting discoveries of the regulatory factors controlling the globin switch. New insights and novel leads for therapeutic strategies based on the pharmacological induction of HbF are discussed. This represents a major breakthrough for rational drug design in the treatment of β-Thal and SCD.

Epigenetic regulation of hemoglobin switching in non-human primates

Human hemoglobin switching describes the highly regulated, sequential expression of the 5 β-like globin genes (HBE, HBG2, HBG1, HBD and HBB) of the human β-globin gene complex. The sequential activation of these β or β-like globin genes during human development from early embryonic through late fetal ('adult') stages, and during erythroid maturation, occurs in an order corresponding to their 5' to 3' location on chromosome 11. The β-hemoglobinopathies are the most common inherited diseases in humanity, and are diseases of mutated HBB or its altered regulation. Since the other β-like globin genes can potentially substitute for defective HBB, much translational research is directed toward understanding and manipulating sequential activation at the human β-globin gene complex to treat β-hemoglobinopathies. Non-human primates provide a vital contribution to such efforts because of their recapitulation of the developmental/maturational switch in hemoglobin production as observed in humans (mice do not model this switch). Valuable insights into druggable epigenetic forces that mediate the switch have been thereby gained. We review important lessons learned in non-human primates, complemented by other studies, and suggest rational next steps.

Targeted Protein Degradation as a Promising Tool for Epigenetic Upregulation of Fetal Hemoglobin

The level of fetal hemoglobin (HbF) is an important disease modifier for β-thalassemia and sickle cell disease patients. Indeed, genetic tinkering with the HbF repression machinery has demonstrated great potential for disease mitigation. Such genetic treatments are costly and the high incidence of β-hemoglobinopathies in low-income countries, therefore, calls for the development of affordable, off-the-shelf, oral treatments. The use of PROTAC (PRoteolysis TArgeting Chimeras) technology to influence the epigenetic mechanisms involved in HbF suppression may provide a solution. In this minireview, we briefly explain the HbF repression network highlighting the epigenetic factors that could be targeted for degradation by PROTACs. We hope that this review will inspire clinicians, molecular and chemical biologists to collaborate and contribute to this fascinating field, which should ultimately deliver drugs that reactivate HbF expression with high specificity and low toxicity.

Fetal hemoglobin in sickle cell anemia

Fetal hemoglobin (HbF) can blunt the pathophysiology, temper the clinical course, and offer prospects for curative therapy of sickle cell disease. This review focuses on (1) HbF quantitative trait loci and the geography of β-globin gene haplotypes, especially those found in the Middle East; (2) how HbF might differentially impact the pathophysiology and many subphenotypes of sickle cell disease; (3) clinical implications of person-to-person variation in the distribution of HbF among HbF-containing erythrocytes; and (4) reactivation of HbF gene expression using both pharmacologic and cell-based therapeutic approaches. A confluence of detailed understanding of the molecular basis of HbF gene expression, coupled with the ability to precisely target by genomic editing most areas of the genome, is producing important preliminary therapeutic results that could provide new options for cell-based therapeutics with curative intent.

Genome editing strategies for fetal hemoglobin induction in beta-hemoglobinopathies

Genome editing to correct a defective β-globin gene or induce fetal globin (HbF) for patients with beta-hemoglobinopathies has the potential to be a curative strategy available to all. HbF reactivation has long been an area of intense interest given the HbF inhibition of sickle hemoglobin (HbS) polymerization. Patients with HbS who also have high HbF tend to have less severe or even minimal clinical manifestations. Approaches to genetically engineer high HbF include de novo generation of naturally occurring hereditary persistence of fetal hemoglobin (HPFH) mutations, editing of transcriptional HbF repressors or their binding sites and/or regulating epigenetic intermediates controlling HbF expression. Recent preclinical and early clinical trial data show encouraging results; however, long-term follow-up is lacking, and the safety and efficacy concerns of genome editing remain.

Epigenetic Reexpression of Hemoglobin F Using Reversible LSD1 Inhibitors: Potential Therapies for Sickle Cell Disease

Sickle cell disease (SCD) is caused by a single nucleotide polymorphism on chromosome 11 in the β-globin gene. The resulting mutant hemoglobin S (HbS) is a poor oxygen transporter and causes a variety of vascular symptoms and organ failures. At birth, the DRED epigenetic complex forms and silences the γ-globin gene, and fetal hemoglobin (HbF, 2 α-, and 2 γ-subunits) is replaced by adult HbA (α2β2) or HbS (α2βs 2) in SCD patients. HbF is a potent inhibitor of HbS polymerization, thus alleviating the symptoms of SCD. The current therapy, hydroxyurea (HU), increases γ-globin and the HbF content in sickle cells but is highly underutilized due to concern for adverse effects and other complications. The DRED complex contains the epigenetic eraser lysine-specific demethylase 1 (LSD1), which appears to serve as a scaffolding protein. Our recently discovered 1,2,4-triazole derivatives and cyclic peptide LSD1 inhibitors promote the upregulation of γ-globin production in vitro without significant toxicity. Herein, we demonstrate that these LSD1 inhibitors can be used to disrupt the DRED complex and increase the cellular HbF content in vitro and in vivo. This approach could lead to an innovative and effective treatment for SCD.

Small molecule therapeutics to treat the β-globinopathies

PURPOSE OF REVIEW

The current review focuses on recent insights into the development of small molecule therapeutics to treat the β-globinopathies.

RECENT FINDINGS

Recent studies of fetal γ-globin gene regulation reveal multiple insights into how γ-globin gene reactivation may lead to novel treatment for β-globinopathies.

SUMMARY

We summarize current information regarding the binding of transcription factors that appear to be impeded or augmented by different hereditary persistence of fetal hemoglobin (HPFH) mutations. As transcription factors have historically proven to be difficult to target for therapeutic purposes, we next address the contributions of protein complexes associated with these HPFH mutation-affected transcription factors with the aim of defining proteins that might provide additional targets for chemical molecules to inactivate the corepressors. Among the enzymes associated with the transcription factor complexes, a group of corepressors with currently available inhibitors were initially thought to be good candidates for potential therapeutic purposes. We discuss possibilities for pharmacological inhibition of these corepressor enzymes that might significantly reactivate fetal γ-globin gene expression. Finally, we summarize the current clinical trial data regarding the inhibition of select corepressor proteins for the treatment of sickle cell disease and β-thalassemia.

δ-Aminolevulinate induces fetal hemoglobin expression by enhancing cellular heme biosynthesis

Sickle cell disease (SCD) and β-thalassemia are inherited blood disorders caused by genetic defects in the β-globin gene on chromosome 11, producing severe disease in people worldwide. Induction of fetal hemoglobin consisting of two α-globin and two γ-globin chains ameliorates the clinical symptoms of both disorders. In the present study, we investigated the ability of δ-aminolevulinate (ALA), the heme precursor, to activate γ-globin gene expression as well as its effects on cellular functions in erythroid cell systems. We demonstrated that ALA induced γ-globin expression at both the transcriptional and protein levels in the KU812 erythroid cell line. Using inhibitors targeting two enzymes in the heme biosynthesis pathway, we showed that cellular heme biosynthesis was involved in ALA-mediated γ-globin activation. Moreover, the transcription factor NRF2 (nuclear factor [erythroid-derived 2]-like 2), a critical regulator of the cellular antioxidant response, was activated by ALA and contributed to mechanisms of γ-globin activation; ALA did not affect cell proliferation and was not toxic to cells. Subsequent studies demonstrated ALA-induced γ-globin activation in erythroid progenitors generated from normal human CD34+ stem cells. These data support future study to explore the potential of stimulating intracellular heme biosynthesis by ALA or similar compounds as a novel therapeutic strategy for treating SCD and β-thalassemia.

Impact statement

Inherited mutations in the β-globin-like genes result in the most common forms of genetic blood disease including sickle cell disease (SCD) and β-thalassemia worldwide. Therefore, effective inexpensive therapies that can be distributed widely are highly desirable. Currently, drug-mediated fetal hemoglobin (HbF) induction can ameliorate clinical symptoms of SCD and β-thalassemia and is the most effective strategy for developing new therapeutic options. In the current study, we confirmed that δ-Aminolevulinate (ALA), the precursor of heme, induces γ-globin expression at both the transcriptional and translational levels in primary human erythroid progenitors. Moreover, the results indicate activation of the transcription factor NRF2 (nuclear factor (erythroid-derived 2)-like 2) by ALA to enhance HbF expression. These data support future study to explore the potential of stimulating intracellular heme biosynthesis by ALA or similar compounds as a novel therapeutic strategy for treating SCD and β-thalassemia.

Targeting sickle cell disease root-cause pathophysiology with small molecules

The complex, frequently devastating, multi-organ pathophysiology of sickle cell disease has a single root cause: polymerization of deoxygenated sickle hemoglobin. A logical approach to disease modification is, therefore, to interdict this root cause. Ideally, such interdiction would utilize small molecules that are practical and accessible for worldwide application. Two types of such small molecule strategies are actively being evaluated in the clinic. The first strategy intends to shift red blood cell precursor hemoglobin manufacturing away from sickle hemoglobin and towards fetal hemoglobin, which inhibits sickle hemoglobin polymerization by a number of mechanisms. The second strategy intends to chemically modify sickle hemoglobin directly in order to inhibit its polymerization. Important lessons have been learnt from the pre-clinical and clinical evaluations to date. Open questions remain, but this review summarizes the valuable experience and knowledge already gained, which can guide ongoing and future efforts for molecular mechanism-based, practical and accessible disease modification of sickle cell disease.

BAP1 regulation of the key adaptor protein NCoR1 is critical for γ-globin gene repression

Human globin gene production transcriptionally "switches" from fetal to adult synthesis shortly after birth and is controlled by macromolecular complexes that enhance or suppress transcription by cis elements scattered throughout the locus. The DRED (direct repeat erythroid-definitive) repressor is recruited to the ε-globin and γ-globin promoters by the orphan nuclear receptors TR2 (NR2C1) and TR4 (NR2C2) to engender their silencing in adult erythroid cells. Here we found that nuclear receptor corepressor-1 (NCoR1) is a critical component of DRED that acts as a scaffold to unite the DNA-binding and epigenetic enzyme components (e.g., DNA methyltransferase 1 [DNMT1] and lysine-specific demethylase 1 [LSD1]) that elicit DRED function. We also describe a potent new regulator of γ-globin repression: The deubiquitinase BRCA1-associated protein-1 (BAP1) is a component of the repressor complex whose activity maintains NCoR1 at sites in the β-globin locus, and BAP1 inhibition in erythroid cells massively induces γ-globin synthesis. These data provide new mechanistic insights through the discovery of novel epigenetic enzymes that mediate γ-globin gene repression.

Fetal hemoglobin induction during decitabine treatment of elderly patients with high-risk myelodysplastic syndrome or acute myeloid leukemia: a potential dynamic biomarker of outcome

Hematologic responses to hypomethylating agents are often delayed in patients with myelodysplastic syndrome or acute myeloid leukemia. Fetal hemoglobin is a potential novel bio-marker of response: recently, we demonstrated that a high fetal hemoglobin level prior to decitabine treatment was associated with superior outcome. Here we investigated whether early fetal hemoglobin induction during decitabine treatment also had prognostic value, and studied the potential of decitabine to induce erythroid differentiation and fetal hemoglobin expression in vitro Fetal hemoglobin levels were measured by high-performance liquid chromatography in patients with higher-risk myelodysplastic syndrome (n=16) and acute myeloid leukemia (n=37) before treatment and after each course of decitabine. Levels above 1.0% were considered induced. Patients achieving complete or partial remission as best response had attained a median fetal hemoglobin of 1.9% after two courses of treatment, whereas the median value in patients who did not reach complete or partial remission was 0.8% (P=0.015). Fetal hemoglobin induction after two courses of decitabine treatment was associated with early platelet doubling (P=0.006), and its subsequent decrease with hematologic relapse. In patients with myelodysplastic syndrome, induction of fetal hemoglobin after course 2 of treatment was associated with longer overall survival: median of 22.9 versus 7.3 months in patients with or without induction of fetal hemoglobin, respectively [hazard ratio=0.2 (95% confidence interval: 0.1-0.9); P=0.03]. In vitro decitabine treatment of two bi-potential myeloid leukemia cell lines (K562 and HEL) resulted in induction of an erythroid (not megakaryocytic) differentiation program, and of fetal hemoglobin mRNA and protein, associated with GATA1 gene demethylation and upregulation. In conclusion, fetal hemoglobin may provide a useful dynamic biomarker during hypomethylating agent therapy in patients with myelodysplastic syndrome or acute myeloid leukemia.

Oral administration of the LSD1 inhibitor ORY-3001 increases fetal hemoglobin in sickle cell mice and baboons

Increased levels of fetal hemoglobin (HbF) lessen the severity of symptoms and increase the life span of patients with sickle cell disease (SCD). More effective strategies to increase HbF are needed because the current standard of care, hydroxyurea, is not effective in a significant proportion of patients. Treatment of the millions of patients projected worldwide would best be accomplished with an orally administered drug therapy that increased HbF. LSD1 is a component of corepressor complexes that repress γ-globin gene expression and are a therapeutic target for HbF reactivation. We have shown that subcutaneous administration of RN-1, a pharmacological LSD1 inhibitor, increased γ-globin expression in SCD mice and baboons, which are widely acknowledged as the best animal model in which to test the activity of HbF-inducing drugs. The objective of this investigation was to test the effect of oral administration of a new LSD1 inhibitor, ORY-3001. Oral administration of ORY-3001 to SCD mice (n = 3 groups) increased γ-globin expression, Fetal Hemoglobin (HbF)-containing (F) cells, and F reticulocytes (retics). In normal baboons (n = 7 experiments) treated with ORY-3001, increased F retics, γ-globin chain synthesis, and γ-globin mRNA were observed. Experiments in anemic baboons (n = 2) showed that ORY-3001 increased F retics (PA8695, predose = 24%, postdose = 66.8%; PA8698: predose = 13%, postdose = 93.6%), γ-globin chain synthesis (PA8695: predose = 0.07 γ/γ+β, postdose = 0.20 γ/γ+β; PA8698: predose = 0.02 γ/γ+β, postdose = 0.44 γ/γ+β), and γ-globin mRNA (PA8695: predose = 0.06 γ/γ+β, postdose = 0.18 γ/γ+β; PA8698: predose = 0.03 γ/γ+β, postdose = 0.33 γ/γ+β). We conclude that oral administration of ORY-3001 increases F retics, γ-globin chain synthesis, and γ-globin mRNA in baboons and SCD mice, supporting further efforts toward the development of this drug for SCD therapy.

Potential role of LSD1 inhibitors in the treatment of sickle cell disease: a review of preclinical animal model data

Sickle cell disease (SCD) is caused by a mutation of the β-globin gene (Ingram VM. Nature 180: 326-328, 1957), which triggers the polymerization of deoxygenated sickle hemoglobin (HbS). Approximately 100,000 SCD patients in the United States and millions worldwide (Piel FB, et al. PLoS Med 10: e1001484, 2013) suffer from chronic hemolytic anemia, painful crises, multisystem organ damage, and reduced life expectancy (Rees DC, et al. Lancet 376: 2018-2031, 2010; Serjeant GR. Cold Spring Harb Perspect Med 3: a011783, 2013). Hematopoietic stem cell transplantation can be curative, but the majority of patients do not have a suitable donor (Talano JA, Cairo MS. Eur J Haematol 94: 391-399, 2015). Advanced gene-editing technologies also offer the possibility of a cure (Goodman MA, Malik P. Ther Adv Hematol 7: 302-315, 2016; Lettre G, Bauer DE. Lancet 387: 2554-2564, 2016), but the likelihood that these strategies can be mobilized to treat the large numbers of patients residing in developing countries is remote. A pharmacological treatment to increase fetal hemoglobin (HbF) as a therapy for SCD has been a long-sought goal, because increased levels of HbF (α₂γ₂) inhibit the polymerization of HbS (Poillin WN, et al. Proc Natl Acad Sci USA 90: 5039-5043, 1993; Sunshine HR, et al. J Mol Biol 133: 435-467, 1979) and are associated with reduced symptoms and increased lifespan of SCD patients (Platt OS, et al. N Engl J Med 330: 1639-1644, 1994; Platt OS, et al. N Engl J Med 325: 11-16, 1991). Only two drugs, hydroxyurea and l-glutamine, are approved by the US Food and Drug Administration for treatment of SCD. Hydroxyurea is ineffective at HbF induction in ~50% of patients (Charache S, et al. N Engl J Med 332: 1317-1322, 1995). While polymerization of HbS has been traditionally considered the driving force in the hemolysis of SCD, the excessive reactive oxygen species generated from red blood cells, with further amplification by intravascular hemolysis, also are a major contributor to SCD pathology. This review highlights a new class of drugs, lysine-specific demethylase (LSD1) inhibitors, that induce HbF and reduce reactive oxygen species.

Fetal Hemoglobin Induction by Epigenetic Drugs

Fetal hemoglobin (HbF) inhibits the root cause of sickle pathophysiology, sickle hemoglobin polymerization. Individuals who naturally express high levels of HbF beyond infancy thus receive some protection from sickle complications. To mimic this natural genetic experiment using drugs, one guiding observation was that HbF is increased during recovery of bone marrow from extreme stress. This led to evaluation and approval of the cytotoxic (cell killing) drug hydroxyurea to treat sickle cell disease. Cytotoxic approaches are limited in potency and sustainability, however, since they require hematopoietic reserves sufficient to repeatedly mount recoveries from stress that destroys their counterparts, and such reserves are finite. HbF induction even by stress ultimately involves chromatin remodeling of the gene for HbF (HBG), therefore, a logical alternative approach is to directly inhibit epigenetic enzymes that repress HBG-implicated enzymes include DNA methyltransferase 1, histone deacetylases, lysine demethylase 1, protein arginine methyltransferase 5, euchromatic histone lysine methyltransferase 2 and chromodomain helicase DNA-binding protein 4. Clinical proof-of-principle that this alternative, noncytotoxic approach can generate substantial HbF and total hemoglobin increases has already been generated. Thus, with continued careful attention to fundamental biological and pharmacologic considerations (reviewed herein), there is potential that rational, molecular-targeted, safe and highly potent disease-modifying therapy can be realized for patients with sickle cell disease, with the accessibility and cost-effective properties needed for world-wide effect.

Targeting histone methyltransferase and demethylase in acute myeloid leukemia therapy

Acute myeloid leukemia (AML) is a clonal disorder of myeloid progenitors characterized by the acquisition of chromosomal abnormalities, somatic mutations, and epigenetic changes that determine a consistent degree of biological and clinical heterogeneity. Advances in genomic technologies have increasingly shown the complexity and heterogeneity of genetic and epigenetic alterations in AML. Among the genetic alterations occurring in AML, frequent are the genetic alterations at the level of various genes involved in the epigenetic control of the DNA methylome and histone methylome. In fact, genes involved in DNA demethylation (such as DNMT3A, TET2, IDH1, and IDH2) or histone methylation and demethylation (EZH2, MLL, DOT1L) are frequently mutated in primary and secondary AML. Furthermore, some histone demethylases, such as LSD1, are frequently overexpressed in AML. These observations have strongly supported a major role of dysregulated epigenetic regulatory processes in leukemia onset and development. This conclusion was further supported by the observation that mutations in genes encoding epigenetic modifiers, such as DMT3A, ASXL1, TET2, IDH1, and IDH2, are usually acquired early and are present in the founding leukemic clone. These observations have contributed to development of the idea that targeting epigenetic abnormalities could represent a potentially promising strategy for the development of innovative treatments of AML. In this review, we analyze those proteins and their inhibitors that have already reached the first stages of clinical trials in AML, namely the histone methyltransferase DOT1L, the demethylase LSD1, and the MLL-interacting protein menin.

Evolving treatment paradigms in sickle cell disease

Sickle cell disease (SCD) is an inheritable hemoglobinopathy characterized by polymerization of hemoglobin S in red blood cells resulting in chronic hemolytic anemia, vaso-occlusive painful crisis, and multiorgan damage. In SCD, an increased reactive oxygen species (ROS) generation occurs both inside the red blood cells and inside the vascular lumen, which augment hemolysis and cellular adhesion. This review discusses the evolving body of literature on the role of ROS in the pathophysiology of SCD as well as some emerging therapeutic approaches to SCD with a focus on the reduction of ROS.

Recent progress in understanding and manipulating haemoglobin switching for the haemoglobinopathies

The major β-haemoglobinopathies, sickle cell disease and β-thalassaemia, represent the most common monogenic disorders worldwide and a steadily increasing global disease burden. Allogeneic haematopoietic stem cell transplantation, the only curative therapy, is only applied to a small minority of patients. Common clinical management strategies act mainly downstream of the root causes of disease. The observation that elevated fetal haemoglobin expression ameliorates these disorders has motivated longstanding investigations into the mechanisms of haemoglobin switching. Landmark studies over the last decade have led to the identification of two potent transcriptional repressors of γ-globin, BCL11A and ZBTB7A. These regulators act with additional trans-acting epigenetic repressive complexes, lineage-defining factors and developmental programs to silence fetal haemoglobin by working on cis-acting sequences at the globin gene loci. Rapidly advancing genetic technology is enabling researchers to probe deeply the interplay between the molecular players required for γ-globin (HBG1/HBG2) silencing. Gene therapies may enable permanent cures with autologous modified haematopoietic stem cells that generate persistent fetal haemoglobin expression. Ultimately rational small molecule pharmacotherapies to reactivate HbF could extend benefits widely to patients.

Fetal haemoglobin induction in sickle cell disease

Fetal haemoglobin (HbF, α2γ2) induction has long been an area of investigation, as it is known to ameliorate the clinical complications of sickle cell disease (SCD). Progress in identifying novel HbF-inducing strategies has been stymied by limited understanding of gamma (γ)-globin regulation. Genome-wide association studies (GWAS) have identified variants in BCL11A and HBS1L-MYB that are associated with HbF levels. Functional studies have established the roles of BCL11A, MYB, and KLF1 in γ-globin regulation, but this information has not yielded new pharmacological agents. Several drugs are under investigation in clinical trials as HbF-inducing agents, but hydroxycarbamide remains the only widely used pharmacologic therapy for SCD. Autologous transplant of edited haematopoietic stem cells holds promise as a cure for SCD, either through HbF induction or correction of the causative mutation, but several technical and safety hurdles must be overcome before this therapy can be offered widely, and pharmacological therapies are still needed.

Fetal hemoglobin in sickle cell anemia: The Arab-Indian haplotype and new therapeutic agents

Fetal hemoglobin (HbF) has well-known tempering effects on the symptoms of sickle cell disease and its levels vary among patients with different haplotypes of the sickle hemoglobin gene. Compared with sickle cell anemia haplotypes found in patients of African descent, HbF levels in Saudi and Indian patients with the Arab-Indian (AI) haplotype exceed that in any other haplotype by nearly twofold. Genetic association studies have identified some loci associated with high HbF in the AI haplotype but these observations require functional confirmation. Saudi patients with the Benin haplotype have HbF levels almost twice as high as African patients with this haplotype but this difference is unexplained. Hydroxyurea is still the only FDA approved drug for HbF induction in sickle cell disease. While most patients treated with hydroxyurea have an increase in HbF and some clinical improvement, 10 to 20% of adults show little response to this agent. We review the genetic basis of HbF regulation focusing on sickle cell anemia in Saudi Arabia and discuss new drugs that can induce increased levels of HbF.

SIRT1 activates the expression of fetal hemoglobin genes

High fetal hemoglobin (HbF, α₂ γ₂ ) levels ameliorate the clinical manifestations of sickle cell disease and β thalassemia. The mechanisms that repress HbF expression and silence γ-globin genes in adults are incompletely characterized and only a single HbF inducer, hydroxyurea, is approved for treatment, and only in patients with sickle cell disease. We identified SIRT1, a protein deacetylase, as a new inducer of γ-globin. SIRT1 knockdown decreased, while SIRT1 ectopic expression upregulated γ-globin gene (HBG) expression in primary human erythroid cells and in K562 cells. The small molecule SIRT1 activators SRT2104 and SRT1720 enhanced HBG expression in cord blood human erythroblasts and reactivated silenced HBG in adult human erythroblasts. Furthermore, SIRT1 binds in the β-globin gene cluster locus control region (LCR) and HBG promoters, promotes the looping of the LCR to HBG promoter, and increases the binding of RNA polymerase II and H4K16Ac in the HBG promoter. SIRT1 suppressed the expression of the HBG suppressors BCL11A, KLF1, HDAC1 and HDAC2. Lastly, SIRT1 did not change the proliferation of human erythroid progenitor cells or the expression of differentiation marker CD235a. These data suggest that SIRT1 activates HBG expression through facilitating LCR looping to the HBG promoter, inhibiting the expression of transcriptional suppressors of HBG, and indirectly increasing histone acetylation in the HBG promoter. SIRT1 is a potential therapeutic target for γ-globin gene induction, and small molecule SIRT1 activators might serve as a lead compound for the development of new HbF inducers.

Pharmacological inhibition of LSD1 and mTOR reduces mitochondrial retention and associated ROS levels in the red blood cells of sickle cell disease

Sickle cell disease (SCD), an inherited blood disorder caused by a point mutation that renders hemoglobin susceptible to polymerization when deoxygenated, affects millions of people worldwide. Manifestations of SCD include chronic hemolytic anemia, inflammation, painful vaso-occlusive crises, multisystem organ damage, and reduced life expectancy. Part of SCD pathophysiology is the excessive formation of intracellular reactive oxygen species (ROS) in SCD red blood cells (RBCs), which accelerates their hemolysis. Normal RBC precursors eliminate their mitochondria during the terminal differentiation process. Strikingly, we observed an increased percentage of RBCs retaining mitochondria in SCD patient blood samples compared with healthy individuals. In addition, using an experimental SCD mouse model, we demonstrate that excessive levels of ROS in SCD are associated with this abnormal mitochondrial retention. Interestingly, the LSD1 inhibitor, RN-1, and the mitophagy-inducing agent mammalian target of rapamycin (mTOR) inhibitor, sirolimus, increased RBC lifespan and reduced ROS accumulation in parallel with reducing mitochondria-retaining RBCs in the SCD mouse model. Furthermore, gene expression analysis of SCD mice treated with RN-1 showed increased expression of mitophagy genes. Our findings suggest that reduction of mitochondria-retaining RBCs may provide a new therapeutic approach to preventing excessive ROS in SCD.

Fetal hemoglobin regulation in β-thalassemia: heterogeneity, modifiers and therapeutic approaches

INTRODUCTION

Stress erythropoiesis induces fetal hemoglobin (HbF) expression in β-thalassemias, however the level of expression is highly variable. The last decade has seen dramatic advances in our understanding of the molecular regulators of HbF production and the genetic factors associated with HbF levels, leading to the promise of new methods of the clinical induction of HbF. Areas covered: This article will review the heterogeneity and genetic modifiers of HbF and HbF induction therapy in β-thalassemia. Expert commentary: One promising curative β-thalassemia therapy is to induce HbF synthesis in β-thalassemic erythrocytes to therapeutic levels before clinical symptom occurs. Further understanding of HbF level variation and regulation is needed in order to predict the response from HbF-inducing approaches.

Developing new pharmacotherapeutic approaches to treating sickle-cell disease

Survival for patients with SCD has been prolonged by improvements in supportive care, including vaccinations, antibiotic prophylaxis, and overall medical management, including tra nsfusion. However, there remains only one approved, partially effective drug for sickle cell disease-hydroxyurea (hydroxycarbamide). The world desperately needs better ways of both treating and preventing the recurrent painful vaso-occlusive episodes pathognomonic of sickle cell disease as well as the end-organ damage that still leads inexorably to severely shortened life expectancies throughout the world. Based on accumulating knowledge about how the abnormal red blood cells of sickle cell disease cause the double scourge of acute painful episodes and progressive end-organ damage, both pharmaceutical enterprises and individual investigators are now pursuing multiple new avenues for treating sickle cell disease. As a result, many compounds are in active development, both in preclinical models as well as in phase I, II, and III clinical trials. These agents target many pathophysiologic processes thought to be critical in sickle cell disease, including the chemical and physical behavior of haemoglobin S, cell adhesion, coagulation pathways, platelet activation, inflammatory pathways, and upregulation of haemoglobin F expression. In addition, recent explorations of the genetic variations that predispose to certain types of sickle cell disease-related tissue injury, such as stroke or nephropathy, are expected to lead to identification of drugs targeting the pathways uncovered by such work. Thus, the next five to ten years holds a promise of new treatments for sickle cell disease.