Major advances in the care of individuals with thalassemia have significantly increased life expectancy in this disease. However, the current paradigm of care remains very cumbersome and has a major impact on quality of life of these individuals and their families. Optimal care requires frequent visits to the hospital for transfusions, monitoring and doctor visits, daily intake of medications with attendant side effects, some persistent symptoms of chronic anemia even in transfused patients, and the potential for complications related to the treatment regimen. Individuals lose days at work or in school, have reduced family time, and suffer an ongoing drain on financial resources, not to mention the psychosocial impairment of dealing with a chronic disease. Knowing the inexorable course of the disease, parents and patients with the option of cure by HSCT most often avail of it, despite the known risk of complications described. However, many individuals do not have a matched related donor, and are thus left to manage a complex disease. Individuals with more severe thalassemia intermedia, while on a less demanding regimen, must cope with anemia and the complications related to IE. Therefore, there remains a significant unmet need for individuals with thalassemia—the need for therapies that would affect the course and provide relief from the burden of the disease and its management.

Over the past decade, major advances have been made thanks to technological developments which have opened a new era of targeted agents and gene therapy. Several approaches have been advanced, and many of these are already in advanced stages of clinical trials with impending regulatory approval for clinical application. These are summarized in Fig. 1). A brief description of each emerging modality follows.

Fig1. VARIED APPROACHES FOR NOVEL THERAPEUTIC INTERVENTIONS IN THE TREATMENT OF β-THALASSEMIA. (Modified from Cappellini MD, Motta I. New therapeutic targets in transfusion-dependent and -independent thalassemia. Hematology Am Soc Hematol Educ Program. 2017[1]:278–283.)

Augmentation of fetal Hb production. In the absence of adequate β - globin production, if the excess α globin chains could be bound with γ globin, to produce HbF, this could increase the Hb level and ameliorate IE to a variable extent, thus modifying the pathophysiologic course of the disease.  Hypomethylation of the γ-globin genes can be induced by the drug 5-azacytidine; indeed, short-term administration of this drug produced the predicted effect in vivo.  Despite much subsequent experimental work, it remains unclear whether the effect was attributable to direct stimulation of fetal genes by demethylation or to recruitment and accelerated differentiation of primitive burst-forming unit-erythroid progenitor cells, which have greater potential to produce HbF.  Hydroxyurea has an effect on burst-forming unit-erythroid similar to that of 5-azacytidine and is a safer drug for long-term use. Short-term as well as longer trials with hydroxyurea have been reported in a number of patients with thalassemia. HbF levels frequently increased without a proportionate increase in total Hb level. A small improvement in total Hb level occurred in some patients with thalassemia intermedia but usually does not exceed 1 to 2 g/dL. Most transfusion-dependent patients have shown no clinical benefit to hydroxyurea. Other approaches using butyrate analogues also did not show sustained positive results. These pharmacological approaches have more or less been abandoned. Two novel strategies are now being employed to augment fetal Hb production.

(i) Inhibition of phosphodiesterase 9 (PDE9) by the compound IMR-687. This has been shown in thalassemia mouse models to increase cGMP by inhibiting conversion of cGMP to GMP by PDE9. Early studies in patients with sickle cell disease showed increases in HbF and proportion of F cells. A clinical trial in thalassemia is in progress.

(ii) Blocking the repression of γ -globin gene expression by BCL11A. Novel approaches based on our better understanding of the silencing of γ -globin gene expression by BCL11A have led to the development of gene-editing strategies. BCL11A is a zinc-finger transcriptional repressor active in erythroid cells and in other hematopoietic lineages. Several studies indicate that BCL11A silences γ -globin.  BCL11A does not bind the γ-globin promoter directly. Rather, it binds motifs in the LCR and different intergenic regions in the globin locus that were previously tied with γ -globin repression. Knock down of BCL11A expression results in the reactivation of HbF expression. Gene therapy or gene-editing techniques are used to modify the patient’s own hematopoietic stem cells The corrected stem cells are transferred via an autologous hematopoietic stem cell transplant ( Fig. 2). The patient’s stem cells are collected by pheresis after mobilization from the marrow, purified, and sent to a central facility, where the cells undergo genetic modification. After myeloablative conditioning, the modified stem cells are reinfused. Once these engraft successfully, and begin to produce γ -globin, hemoglobin F containing 2 α -globin chains and 2 γ -globin chains is produced. Success depends on successful editing and engraftment of adequate numbers of stem cells. several approaches have been used and are in clinical trials: (a) introduction of a viral vector encoding an inhibitory RNA (RNAi) that interferes with the ability of BCL11a to interact with its cognate enhance sequence motifs; (b) targeting protein-DNA inter actions using either transcription activator-like effector nucleases (TALENS) or zinc finger nucleases, and (c) targeting RNA-DNA interactions using CRISPR-Cas-9. Early results from this trial in 5 patients with thalassemia have shown successful engraftment of the edited stem cells with excellent expression of HbF, suggesting that this strategy is feasible and could result in durable continued expression of γ -globin, and thus production of HbF.

Fig2. SCHEMA FOR AUTOLOGOUS TRANSPLANTATION OF STEM CELLS MODIFIED BY LENTIVIRAL GENE ADDITION OR CRISPR CAS9 GENE EDITING.

Targeted therapies aimed at ameliorating IE. Several molecules have been/are being developed with the aim of reducing IE and thus not only improving red cell production and reducing anemia, but also possibly preventing progressive bone disease as a result. These include:

(i) Luspatercept: This is a recombinant fusion protein containing a modified extracellular domain of ActRIIB, which binds GDF11 and other TGF- β superfamily ligands, inhibits Smad2/3 signaling, and promotes RBC differentiation/maturation. As a result more proliferating erythroid precursors are able to mature, reducing intramedullary apoptosis, and ameliorating IE. Early studies in normal human volunteers showed a dose-dependent increase in Hb levels, and a good tolerability profile. Phase II studies confirmed that IE was improved in patients with both TDT and NTDT, with clinically meaningful reductions in transfusion requirements. The phase III 48-week BELIEVE study in adults with TDT met all of its primary and secondary endpoints for significant reduction in transfusion requirements by 33% to 50% compared with baseline. An extension phase is ongoing and will inform as to the durability of this therapy, which is now approved for clinical use.

 (ii) Ruxolitinib: This JAK2 inhibitor was developed based on the demonstration that JAK2 played a role mediating IE and its inhibition in the mouse model improved effective erythropoiesis and reduced splenomegaly.  However, the phase 2a study in individuals with TDT did not show a reduction in transfusion requirement, though there was significant reduction in spleen size No further studies using this agent are planned.

(iii) Hepcidin analogs: A clearer understanding of IE in thalassemia and the interaction between erythropoiesis and hepcidin via ERFE led to the use of hepcidin analogs in the mouse model of thalassemia.  Treated mice showed a reduction in spleen size and improvement of anemia.  A potential additional benefit would be to directly inhibit iron absorption and slow the development of iron overload. Further, increasing hepcidin would sequester iron in stores, prevent release, and therefore reduce iron exposure of the heart and endocrine organs. Clinical trials of these analogs have been initiated in humans, looking for reductions in transfusion requirement as the endpoint. In a different study, patients with thalassemia and transfusional iron overload were given hepcidin to study its effect on myocardial iron. However, this trial was terminated after the interim analysis did not show any beneficial effect.

 (iv) AG-348 (Mitapivat): Improving aerobic glycolysis and reducing oxidative stress in developing erythroid precursors could amelio rate apoptosis and IE and increase production of red cells. Further, these red cells may also have a longer lifespan. Mitapivat is an oral, small molecule allosteric activator of wild-type and a variety of mutated pyruvate kinase-R (PKR) enzymes. In non-transfusion dependent individuals with β thalassemia intermedia and HbH disease, early short-term phase II data show an increase in Hb levels and a reduction in markers of hemolysis. This agent is in clinical trials in NTD patients, and trials in TDT are planned.

(v) FT-4202 is another pyruvate kinase activator which has shown benefit in sickle cell disease, with the same mechanism of action postulated as AG348. Clinical trials in thalassemia are also planned.

Correction of the defective globin gene. The most advanced and exciting curative strategy for the thalassemias is aimed at correcting the defective gene. The only reliable conventional curative treatment, allogeneic HSCT from a matched related donor, is available to a limited proportion of patients with such a donor. A gene therapy approach, being autologous, greatly expands the pool of patients who could be cured.

This is done either by adding a new copy of the gene or possibly by correcting the defective gene through CRISPR/Cas-9 splicing. Gene addition strategies are in fairly advanced stages of clinical trials. The first successful gene therapy for β -thalassemia was reported in 2010, when a patient with HbE/ β 0 thalassemia was cured using a lentiviral vector. The corrected stem cells are transferred via an autologous hematopoietic stem cell transplant (see Fig. 2). The patient’s stem cells are collected by pheresis after mobilization from the marrow, purified, and taken to the gene transfer facility. A functional copy of the β -globin gene (or possibly γ -globin gene) is introduced into the stem cells via transfection by a lentiviral vector. After myeloablative conditioning, the modified stem cells are reinfused. Once these engraft successfully, and begin to produce β -globin, hemoglobin A, containing 2 α -globin chains and 2 β -globin (normal or modified) chains is produced. The efficacy of the process depends on successful transfection of the stem cells in adequate numbers, the engraftment of these cells, and the maintenance of a high vector copy number (VCN) per cell to ensure adequate β -globin production. An added benefit of this approach is the virtual absence of graft-versus-host disease in the autologous setting.

Two vectors are currently in trials:

(i) BB305 vector: This vector (Fig.3) encodes a β -globin gene which produces a β -globin chain with a single amino acid substitution—T87Q—resulting in the production of HbAT87Q, thus allowing easy measurement of the gene product as different from endogenous HbA production. Early clinical trials using this vector demonstrated proof of principle early, and a proportion of patients with both non- β 0/ β 0 (and β 0/ β 0 genotypes showed robust production of the HbAT87Q, with transfusion independence, more in the non- β 0/ β 0 patients. Patients who did not respond as well still experienced a reduction in transfusion requirements. Single agent busulfan myeloablative conditioning was used. The therapy was safe, with no deaths or graft failures during the trial, and all complications were expected and similar to those seen from myeloablative conditioning in allogeneic HSCT, including neutropenic fevers, mucositis, and veno-occlusive disease. No graft-versus-host disease was noted. Furthermore, vector integration was polyclonal, and there was no vector-mediated replication- competent lentivirus detected, thus providing reassurance that there was a very low risk for insertional mutagenesis. In addition, there was stable transfection, with a VCN in the 0.6 to 0.7 copies per cell, and stable production of HbAT87Q in most patients. These patients have now been followed for approximately 5 years, and the graft appears durable.

Fig3. CONSTRUCT OF THE BB305 VECTOR USED FOR LENTIVIRAL GENE ADDITION TO HEMATOPOIETIC STEM CELLS FOLLOWED BY AUTOLOGOUS TRANSPLANT.

Clinical trials using the same vector but manufactured with a refined process are currently underway in non- β 0/ β 0 patients and β 0/ β 0 patients, and preliminary results show improved VCNs in the 2 to 3 range, with a correspondingly greater production of HbAT87Q and thus a greater proportion of patients with transfusion independence. These data are being updated regularly, and so far, results are very promising, with success rates similar to those following allogeneic HSCT from a matched related donor. (ii) GLOBE vector: Also a lentiviral vector, this contained a normal human β -globin gene with no modifications. Other differences from the BB305 trials were the use of thiotepa and treosulfan for myeloablative conditioning, and the introduction of the modified cells directly into the BM to avoid passage through, and possible clearance by the spleen. Though still early, the preliminary results from this phase I/II trial have been positive.

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علماء يكتشفون انخفاض صلابة الألماس عند تحويله إلى مقياس نانوي

ثورة في علاج الصمم.. علاج جيني يحقق نتائج استثنائية في استعادة السمع

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المزيد

مواضيع ذات صلة

Thalassemic Structural Variants

Chronic Care of the Adult Patient With Thalassemia : Hematopoietic Stem Cell Transplantation

Survival in Patients With Thalassemia Major

Organ System Manifestations of Hypothyroidism: Haematological Changes

Organ System Manifestations of Hypothyroidism: Musculoskeletal Changes

Organ System Manifestations of Hypothyroidism: Cerebral and neurological Changes

Organ System Manifestations of Hypothyroidism: Respiratory Changes

Organ System Manifestations of Hypothyroidism: Cardiovascular Changes

Organ System Manifestations of Hypothyroidism : Cutaneous Manifestations and Changes in the Connective Tissues

Thromboembolic Conditions

Conditions that Cause Excessive Bleeding in Humans

Absent Intrinsic Factor Secretion and Pernicious Anemia