Current status of globin gene therapy for the treatment of β-thalassaemia

Erythrocytes as a preferential target of oxidative stress in blood

Red blood cells (RBC) are specifically differentiated to transport oxygen and carbon dioxide in the blood and they lack most organelles, including mitochondria. The autoxidation of hemoglobin constitutes a major source of reactive oxygen species (ROS). Nitric oxide, which is produced by endothelial nitric oxide synthase (NOS3) or via the hemoglobin-mediated conversion of nitrite, interacts with ROS and results in the production of reactive nitrogen oxide species. Herein we present an overview of anemic diseases that are closely related to oxidative damage. Because the compensation of proteins by means of gene expression does not proceed in enucleated cells, antioxidative and redox systems play more important roles in maintaining the homeostasis of RBC against oxidative insult compared to ordinary cells. Defects in hemoglobin and enzymes that are involved in energy production and redox reactions largely trigger oxidative damage to RBC. The results of studies using genetically modified mice suggest that antioxidative enzymes, notably superoxide dismutase 1 and peroxiredoxin 2, play essential roles in coping with oxidative damage in erythroid cells, and their absence limits erythropoiesis, the life-span of RBC and consequently results in the development of anemia. The degeneration of the machinery involved in the proteolytic removal of damaged proteins appears to be associated with hemolytic events. The ubiquitin-proteasome system is the dominant machinery, not only for the proteolytic removal of damaged proteins in erythroid cells but also for the development of erythropoiesis. Hence, despite the fact that it is less abundant in RBC compared to ordinary cells, the aberrant ubiquitin-proteasome system may be associated with the development of anemic diseases via the accumulation of damaged proteins, as typified in sickle cell disease, and impaired erythropoiesis.

β-Globin Lentiviral Vectors Have Reduced Titers due to Incomplete Vector RNA Genomes and Lowered Virion Production

Lentiviral vectors (LVs) commonly used for the treatment of hemoglobinopathies often have low titers and sub-optimal gene transfer efficiency for human hematopoietic stem and progenitor cells (HSPCs), hindering clinical translation and commercialization for ex vivo gene therapy. We observed that a high percentage of β-globin LV viral genomic RNAs were incomplete toward the 3′ end in packaging cells and in released vector particles. The incomplete vector genomes impeded reverse transcription in target cells, limiting stable gene transfer to HSPCs. By combining three modifications to vector design and production (shortening the vector length to 5.3 kb; expressing HIV-1 Tat protein during packaging; and packaging in PKR−/− cells) there was a 30-fold increase in vector titer and a 3-fold increase in vector infectivity in HSPCs. These approaches may improve the manufacturing of β-globin and other complex LVs for enhanced gene delivery and may facilitate clinical applications.

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Vector genomes are truncated in a length-dependent manner during packaging

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Truncated RNAs cannot be reverse transcribed, impeding titer and infectivity

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Protein kinase R inhibits virion formation for bidirectional lentiviral vectors

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Three strategies to improve lentiviral vector titer by 30× and infectivity by 3×

Synthetic Biology--Toward Therapeutic Solutions

Higher multicellular organisms have evolved sophisticated intracellular and intercellular biological networks that enable cell growth and survival to fulfill an organism's needs. Although such networks allow the assembly of complex tissues and even provide healing and protective capabilities, malfunctioning cells can have severe consequences for an organism's survival. In humans, such events can result in severe disorders and diseases, including metabolic and immunological disorders, as well as cancer. Dominating the therapeutic frontier for these potentially lethal disorders, cell and gene therapies aim to relieve or eliminate patient suffering by restoring the function of damaged, diseased, and aging cells and tissues via the introduction of healthy cells or alternative genes. However, despite recent success, these efforts have yet to achieve sufficient therapeutic effects, and further work is needed to ensure the safe and precise control of transgene expression and cellular processes. In this review, we describe the biological tools and devices that are at the forefront of synthetic biology and discuss their potential to advance the specificity, efficiency, and safety of the current generation of cell and gene therapies, including how they can be used to confer curative effects that far surpass those of conventional therapeutics. We also highlight the current therapeutic delivery tools and the current limitations that hamper their use in human applications.

Development and Recent Progresses of Gene Therapy for β-Thalassemia

β-thalassemias are among the most common inherited monogenic disorders worldwide due to mutations in the β-globin gene that reduce or abolish the production of the β-globin chain resulting in transfusion-dependent chronic anemia. Currently, the only curative treatment is allogeneic hematopoietic stem cells (HSCs) transplantation, but this option is limited by the a vailability of HLA-matched donor. Gene therapy, based on autologous transplantation of genetically corrected HSCs, holds the promise to treat patients lacking a compati ble bone marrow donor. I nit ial attempts of gene transfer have been unsuccessful due to limitations of available vectors to stably transfer a globin gene in HSCs and reach high and regulated expression in the erythroid progeny. With the advent of lentiviral vectors (LVs), based on human immunodeficiency virus, many of the initial limitations have been overcome. Since 2000 when Sadelain and co-workers first demonstrated successful globin gene transfer in murine thalassemia models with improvement of the phenotype using a recombinant β globin/LV, several other groups have developed different vectors encoding either β, γ or mutated globin genes and confirmed these results in both murine models and erythroid progeny derived from patient’s HSCs. In light of these encouraging results, research has recently moved into clinical trials that are ongoing or soon to begin. One participant in an ongoing gene transfer trial for β-thalassemia has achieved clinical benefit with elimination of his transfusi on re quirement. Here , dev elopmen t and recent progress of gene therapy for β-thalassemia is reviewed.

Combining gene therapy and fetal hemoglobin induction for treatment of β-thalassemia

β-thalassemias are caused by nearly 300 mutations of the β-globin gene, leading to a low or absent production of adult hemoglobin (HbA). Two major therapeutic approaches have recently been proposed: gene therapy and induction of fetal hemoglobin (HbF) with the objective of achieving clinically relevant levels of Hbs. The objective of this article is to describe the development of therapeutic strategies based on a combination of gene therapy and induction of HbFs. An increase of β-globin gene expression in β-thalassemia cells can be achieved by gene therapy, although de novo production of clinically relevant levels of adult Hb may be difficult to obtain. On the other hand, an increased production of HbF is beneficial in β-thalassemia. The combination of gene therapy and HbF induction appears to be a pertinent strategy to achieve clinically relevant results.

β-globin gene transfer to human bone marrow for sickle cell disease

Autologous hematopoietic stem cell gene therapy is an approach to treating sickle cell disease (SCD) patients that may result in lower morbidity than allogeneic transplantation. We examined the potential of a lentiviral vector (LV) (CCL-βAS3-FB) encoding a human hemoglobin (HBB) gene engineered to impede sickle hemoglobin polymerization (HBBAS3) to transduce human BM CD34+ cells from SCD donors and prevent sickling of red blood cells produced by in vitro differentiation. The CCL-βAS3-FB LV transduced BM CD34+ cells from either healthy or SCD donors at similar levels, based on quantitative PCR and colony-forming unit progenitor analysis. Consistent expression of HBBAS3 mRNA and HbAS3 protein compromised a fourth of the total β-globin-like transcripts and hemoglobin (Hb) tetramers. Upon deoxygenation, a lower percentage of HBBAS3-transduced red blood cells exhibited sickling compared with mock-transduced cells from sickle donors. Transduced BM CD34+ cells were transplanted into immunodeficient mice, and the human cells recovered after 2-3 months were cultured for erythroid differentiation, which showed levels of HBBAS3 mRNA similar to those seen in the CD34+ cells that were directly differentiated in vitro. These results demonstrate that the CCL-βAS3-FB LV is capable of efficient transfer and consistent expression of an effective anti-sickling β-globin gene in human SCD BM CD34+ progenitor cells, improving physiologic parameters of the resulting red blood cells.

Alternative options for DNA-based experimental therapy of β-thalassemia

INTRODUCTION

Beta-thalassemias are caused by more than 200 mutations of the β-globin gene, leading to low or absent production of adult hemoglobin. Achievements have been made with innovative therapeutic strategies for β-thalassemias, based on research conducted at the levels of gene structure, transcription, mRNA processing and protein synthesis.

AREAS COVERED

The objective of this review is to describe the development of therapeutic strategies employing viral and non-viral DNA-based approaches for treatment of β-thalassemia.

EXPERT OPINION

Modification of β-globin gene expression in β-thalassemia cells has been achieved by gene therapy, correction of the mutated β-globin gene and RNA repair. In addition, cellular therapy has been proposed for β-thalassemia, including reprogramming of somatic cells to generate induced pluripotent stem cells to be genetically corrected. Based on the concept that increased production of fetal hemoglobin (HbF) is beneficial in β-thalassemia, DNA-based approaches to increase HbF production have been optimized, including treatment of target cells with lentiviral vectors carrying γ-globin genes. Finally, DNA-based targeting of α-globin gene expression has been applied to reduce the excess of α-globin production by β-thalassemia cells, one of the major causes of the clinical phenotype.

The β-globin locus control region in combination with the EF1α short promoter allows enhanced lentiviral vector-mediated erythroid gene expression with conserved multilineage activity

Some gene therapy strategies are compromised by the levels of gene expression required for therapeutic benefit, and also by the breadth of cell types that require correction. We designed a lentiviral vector system in which a transgene is under the transcriptional control of the short form of constitutively acting elongation factor 1α promoter (EFS) combined with essential elements of the locus control region of the β-globin gene (β-LCR). We show that the β-LCR can upregulate EFS activity specifically in erythroid cells but does not alter EFS activity in myeloid or lymphoid cells. Experiments using the green fluorescent protein (GFP) reporter or the human adenosine deaminase (ADA) gene demonstrate 3-7 times upregulation in vitro but >20 times erythroid-specific upregulation in vivo, the effects of which were sustained for 1 year. The addition of the β-LCR did not alter the mutagenic potential of the vector in in vitro mutagenesis (IM) assays although microarray analysis showed that the β-LCR upregulates ~9% of neighboring genes. This vector design therefore combines the benefits of multilineage gene expression with high-level erythroid expression, and has considerable potential for correction of multisystem diseases including certain lysosomal storage diseases through a hematopoietic stem cell (HSC) gene therapy approach.

Allogeneic cellular and autologous stem cell therapy for sickle cell disease: 'whom, when and how'

Sickle cell disease (SCD) is an autosomal recessive inherited hematological disorder characterized by chronic hemolysis and vaso-occlusion, resulting in multiorgan dysfunction and premature death. The only known curative therapy for patients with severe SCD is myeloablative conditioning and allo-SCT from HLA-matched sibling donors. In this state of the art review, we discuss current and future considerations including patient selection/eligibility, intensity of conditioning regimens, allogeneic graft sources, graft manipulation, mixed donor chimerism, organ function and stability and autologous gene correction stem cell strategies. Recent novel approaches to promote mixed donor chimerism have included the use of matched unrelated adult donors, umbilical cord blood donors, haploidentical familial donors and the utilization of nonmyeloablative, such as reduced intensity and reduced toxicity conditioning regimens. Future strategies will include gene therapy and autologous gene correction stem cell designs. Prospects are bright for novel stem and cellular approaches for patients with severe SCD, and we are currently at the end of the beginning for utilizing cellular therapeutics for the curative treatment of this chronic and debilitating condition.

Recombinant AAV2-mediated β-globin expression in human fetal hematopoietic cells from the aborted fetuses with β-thalassemia major

Genetic correction of autologous hematopoietic stem cells has been proposed as an attractive treatment method for β-thalassemia. Our previous study has shown that recombinant adeno-associated virus 2 (rAAV2) efficiently transduces human fetal liver hematopoietic cells, and mediates the expression of the human β-globin gene in vivo. In this study, we investigated whether rAAV2 could also mediate the expression of normal β-globin gene in human hematopoietic cells from β-thalassemia patients. Human hematopoietic cells were isolated from aborted β-thalassemia major fetuses, transduced with rAAV2-β-globin, and then transplanted into nude mice. We found that rAAV2-β-globin transduced human fetal hematopoietic cells, as determined by allele-specific PCR analysis. Furthermore, β-globin transgene expression was detected in human hematopoietic cells up to 70 days post-transplantation in the recipient mice. High-pressure liquid chromatography analysis showed that human β-globin expression levels increased significantly compared with control, as indicated by a 1.2-2.8-fold increase in the ratio of β/α-globin chain. These novel data demonstrate that rAAV2 can transduce and mediate the normal β-globin gene expression in fetal hematopoietic cells from β-thalassemia patients. Our findings further support the potential use of rAAV-based gene therapy in the treatment of human β-thalassemia.

Thalassemia: an overview of 50 years of clinical research

The thalassemias are attributable to the defective production of the α- and β-globin polypeptides of hemoglobin. Significant discoveries have illuminated the pathophysiology and enhanced the prevention and treatment of the thalassemias, and this article reviews many of the advances that have occurred in the past 50 years. However, the application of new approaches to the treatment of these disorders has been slow, particularly in the developing world where the diseases are common, but there is definite progress. This article emphasizes how the increasing knowledge of cellular and molecular biology are facilitating the development of more effective therapies for these patients.

Genetic predisposition to β-thalassemia and sickle cell anemia in Turkey: a molecular diagnostic approach

The thalassemia syndromes are a diverse group of inherited disorders that can be characterized according to their insufficient synthesis or absent production of one or more of the globin chains. They are classified in to α, β, γ, δβ, δ, and εγδβ thalassemias depending on the globin chain(s) affected. The β-thalassemias refer to that group of inherited hemoglobin disorders, which are characterized by a reduced synthesis (β(+)-thalassemia) or absence (β(0)-thalassemia) of beta globin (β-globin) chain production (1). Though known as single-gene disorders, hemoglobinopathies such as β-thalassemia and sickle cell anemia are far from being fully resolved in terms of cure, considering the less complex nature of the beta globin (β-globin) gene family compared to more complex multifactorial genetic disorders such as cancer. Currently, there are no definitive therapeutic options for patients with β-thalassemia and sickle cell anemia, and new insights into the pathogenesis of these devastating diseases are urgently needed. Here we address in detail the overall picture utilizing molecular diagnostic approaches that contribute to unraveling the population-specific mutational analysis of β-globin gene. We also present approaches for molecular diagnostic strategies that are applicable to β-thalassemia, sickle cell anemia, and other genetic disorders.

Physiological and tissue-specific vectors for treatment of inherited diseases

After more than 1500 gene therapy clinical trials in the past two decades, the overall conclusion is that for gene therapy (GT) to be successful, the vector systems must still be improved in terms of delivery, expression and safety. The recent development of more efficient and stable vector systems has created great expectations for the future of GT. Impressive results were obtained in three primary immunodeficiencies and other inherited diseases such as congenital blindness, adrenoleukodystrophy or junctional epidermolysis bullosa. However, the development of leukemia in five children included in the GT clinical trials for X-linked severe combined immunodeficiency and the silencing of the therapeutic gene in the chronic granulomatous disease clearly showed the importance of improving safety and efficiency. In this review, we focus on the main strategies available to achieve physiological or tissue-specific expression of therapeutic transgenes and discuss the importance of controlling transgene expression to improve safety. We propose that tissue-specific and/or physiological viral vectors offer the best balance between efficiency and safety and will be the tools of choice for future clinical trials in GT of inherited diseases.

Gene therapy for β-thalassaemia: the continuing challenge

The β-thalassaemias are inherited anaemias that form the most common class of monogenic disorders in the world. Treatment options are limited, with allogeneic haematopoietic stem cell transplantation offering the only hope for lifelong cure. However, this option is not available for many patients as a result of either the lack of compatible donors or the increased risk of transplant-related mortality in subjects with organ damage resulting from accumulated iron. The paucity of alternative treatments for patients that fall into either of these categories has led to the development of a revolutionary treatment strategy based on gene therapy. This approach involves replacing allogeneic stem cell transplantation with the transfer of normal globin genes into patient-derived, autologous haematopoietic stem cells. This highly attractive strategy offers several advantages, including bypassing the need for allogeneic donors and the immunosuppression required to achieve engraftment of the transplanted cells and to eliminate the risk of donor-related graft-versus-host disease. This review discusses the many advances that have been made towards this endeavour as well as the hurdles that must still be overcome before gene therapy for β-thalassaemia, as well as many other gene therapy applications, can be widely applied in the clinic.

Strategy for a multicenter phase I clinical trial to evaluate globin gene transfer in beta-thalassemia

Globin gene transfer in autologous hematopoietic stem cells offers a potentially curative treatment option for patients suffering from beta-thalassemia major who lack an HLA-matched hematopoietic stem cell donor. Based on extensive preclinical investigation, we are initiating a phase I clinical trial using G-CSF mobilized, autologous CD34(+) cells transduced with a vector similar to the original TNS9 vector. Our first mobilizations in adult beta-thalassemic subjects have been well tolerated and yielded the required CD34(+) cell dose. To minimize toxicity to enrolled subjects, and in the absence of a demonstrated requirement for myeloablative conditioning, our trial will use a reduced intensity conditioning regimen. Because low vector titers may adversely affect efficacy and safety, we have focused on vector manufacturing processes. We are now in a position to transfer our globin lentiviral vectors in a clinically relevant dosage (averaging 0.8 vector copy per cell in bulk CD34(+) cells) and to supply clinical grade vector to collaborating centers in the U.S.A. and in Europe. We anticipate that the first U.S. trial of globin gene transfer will start in 2010.

Recent advances in lentiviral vector development and applications

Lentiviral vectors (LVs) have emerged as potent and versatile vectors for ex vivo or in vivo gene transfer into dividing and nondividing cells. Robust phenotypic correction of diseases in mouse models has been achieved paving the way toward the first clinical trials. LVs can deliver genes ex vivo into bona fide stem cells, particularly hematopoietic stem cells, allowing for stable transgene expression upon hematopoietic reconstitution. They are also useful to generate induced pluripotent stem cells. LVs can be pseudotyped with distinct viral envelopes that influence vector tropism and transduction efficiency. Targetable LVs can be generated by incorporating specific ligands or antibodies into the vector envelope. Immune responses toward the transgene products and transduced cells can be repressed using microRNA-regulated vectors. Though there are safety concerns regarding insertional mutagenesis, their integration profile seems more favorable than that of gamma-retroviral vectors (gamma-RVs). Moreover, it is possible to minimize this risk by modifying the vector design or by employing integration-deficient LVs. In conjunction with zinc-finger nuclease technology, LVs allow for site-specific gene correction or addition in predefined chromosomal loci. These recent advances underscore the improved safety and efficacy of LVs with important implications for clinical trials.

Musings on genome medicine: enzyme-replacement therapy of the lysosomal storage diseases

The lysosomal storage diseases, such as Gaucher's disease, mucopolysaccharidosis I, II and IV, Fabry's disease, and Pompe's disease, are rare inherited disorders whose symptoms result from enzyme deficiency causing lysosomal accumulation. Until effective gene-replacement therapy is developed, expensive, and at best incomplete, enzyme-replacement therapy is the only hope for sufferers of rare lysosomal storage diseases. Preventive strategies involving carrier detection should be a priority toward the successful management of these conditions.

Supplying clotting factors from hematopoietic stem cell-derived erythroid and megakaryocytic lineage cells

Systemically distributed proteins such as clotting factors have been traditionally expressed from muscle or liver to achieve therapeutic, long-term expression. As long-lived cell capable of generating an abundant progeny, hematopoietic stem cells (HSCs) also merit consideration for this purpose. To be clinically relevant, this approach would require that hematopoietic cells be capable of expressing high levels of functional, secreted proteins, that the risk of insertional oncogenesis be minimized, and that sufficient stem cell engraftment be achieved following minimally invasive conditioning. Recent reports demonstrate the feasibility of expressing either factor IX (FIX) or factor VIII (FVIII) in erythroblasts and platelets using lineage-restricted vectors, resulting in effective treatments in mouse models of hemophilia. The erythroid system is especially powerful in providing high protein output, yielding FIX levels approaching 1 micro g/ml per vector copy in the plasma of long-term hematopoietic chimeras, a secretion level that vastly outperforms any other current mammalian constitutive or long-terminal repeat (LTR)-driven vector system. These early but promising studies raise the prospect of further developing these strategies for clinical investigation.

Induction of endogenous gamma-globin gene expression with decoy oligonucleotide targeting Oct-1 transcription factor consensus sequence

Human β-globin disorders are relatively common genetic diseases cause by mutations in the β-globin gene. Increasing the expression of the γ-globin gene has great benefits in reducing complications associated with these diseases. The Oct-1 transcription factor is involved in the transcriptional regulation of the γ-globin gene. The human γ-globin genes (both Aγ and Gγ-globin genes) carry three Oct-1 transcription factor consensus sequences within their promoter regions. We have studied the possibility of inducing γ-globin gene expression using decoy oligonucleotides that target the Oct-1 transcription factor consensus sequence. A double-stranded 22 bp decoy oligonucleotide containing the Oct-1 consensus sequence was synthesized. The results obtained from our in vitro binding assay revealed a strong competitive binding of the decoy oligonucleotide for the Oct-1 transcription factor. When K562 human erythroleukemia cells were treated with the Oct-1 decoy oligonucleotide, significant increases in the level of the γ-globin mRNA were observed. The results of our western blots further demonstrated significant increases of the fetal hemoglobin (HbF, α2γ2) in the Oct-1 decoy oligonucleotide-treated K562 cells. The results of our immunoprecipitation (IP) studies revealed that the treatment of K562 cells with the Oct-1 decoy oligonucleotide significantly reduced the level of the endogenous γ-globin gene promoter region DNA co-precipitated with the Oct-1 transcription factor. These results suggest that the decoy oligonucleotide designed for the Oct-1 transcription factor consensus sequence could induce expression of the endogenous γ-globin gene through competitive binding of the Oct-1 transcription factor, resulting in activation of the γ-globin genes. Therefore, disrupting the bindings of the Oct-1 transcriptional factors with the decoy oligonucleotide provides a novel approach for inducing expression of the γ-globin genes. It also provides an innovative strategy for the treatment of many disease conditions, including sickle cell anemia and β-thalassemia.

Hematopoietic stem cell gene transfer for the treatment of hemoglobin disorders

Hematopoietic stem cell (HSC)-targeted gene transfer is an attractive approach for the treatment of a number of hematopoietic disorders caused by single gene defects. Indeed, in a series of gene transfer trials for two different primary immunodeficiencies beginning early in this decade, outstanding success has been achieved. Despite generally low levels of engrafted, genetically modified HSCs, these trials were successful because of the marked selective advantage of gene-corrected lymphoid precursors that allowed reconstitution of the immune system. Unlike the immunodeficiencies, this robust level of in vivo selection is not available to hematopoietic repopulating cells or early progenitor cells following gene transfer of a therapeutic globin gene in the setting of beta-thalassemia and sickle cell disease. Both preclinical and clinical transplant studies involving bone marrow chimeras suggest that 20% or higher levels of engraftment of genetically modified HSCs will be needed for clinical success in the most severe of these disorders. Encouragingly, gene transfer levels in this range have recently been reported in a lentiviral vector gene transfer clinical trial for children with adrenoleukodystrophy. A clinical gene transfer trial for beta-thalassemia has begun in France, and one patient with transfusion-dependent HbE/beta-thalassemia has demonstrated a therapeutic effect after transplantation with autologous CD34(+) cells genetically modified with a beta-globin lentiviral vector. Here, the development and recent progress of gene therapy for the hemoglobin disorders is reviewed.

Hemoglobin research and the origins of molecular medicine

Much of our understanding of human physiology, and of many aspects of pathology, has its antecedents in laboratory and clinical studies of hemoglobin. Over the last century, knowledge of the genetics, functions, and diseases of the hemoglobin proteins has been refined to the molecular level by analyses of their crystallographic structures and by cloning and sequencing of their genes and surrounding DNA. In the last few decades, research has opened up new paradigms for hemoglobin related to processes such as its role in the transport of nitric oxide and the complex developmental control of the alpha-like and beta-like globin gene clusters. It is noteworthy that this recent work has had implications for understanding and treating the prevalent diseases of hemoglobin, especially the use of hydroxyurea to elevate fetal hemoglobin in sickle cell disease. It is likely that current research will also have significant clinical implications, as well as lessons for other aspects of molecular medicine, the origin of which can be largely traced to this research tradition.

Fetal hemoglobin chemical inducers for treatment of hemoglobinopathies

The switch from fetal ((G)gamma and (A)gamma) to adult (beta and delta) globin gene expression occurs at birth, leading to the gradual replacement of HbF with HbA. Genetic regulation of this switch has been studied for decades, and the molecular mechanisms underlying this developmental change in gene expression have been in part elucidated. The understanding of the developmental regulation of gamma-globin gene expression was paralleled by the identification of a series of chemical compounds able to reactivate HbF synthesis in vitro and in vivo in adult erythroid cells. Reactivation of HbF expression is an important therapeutic option in patients with hemoglobin disorders, such as sickle cell anemia and beta-thalassemia. These HbF inducers can be grouped in several classes based on their chemical structures and mechanisms of action. Clinical studies with some of these agents have shown that they were effective, in a part of patients, in ameliorating the clinical condition. The increase in HbF in response to these drugs varies among patients with beta-thalassemia and sickle cell disease due to individual genetic determinants.