Current therapeutic landscape of β-thalassemia: focus on gene therapy

Abstract

β-thalassemia is an inherited blood disorder characterized by chronic anemia, ineffective erythropoiesis, and in its most severe form, lifelong transfusion dependence. The standard of care for transfusion-dependent thalassemia (TDT) is regular red blood cell transfusions to relieve the anemia and suppress ineffective erythropoiesis and iron chelation therapy to mitigate morbidity and mortality related to iron overload. Allogeneic hematopoietic stem cell transplantation is a curative option but is only available to patients with an appropriate donor and carries risks of graft-versus-host disease and other transplant-related morbidity. In recent years, the therapeutic landscape for TDT has changed dramatically with the approval of two autologous gene therapies in the United States: betibeglogene autotemcel (beti-cel) and exagamglogene autotemcel (exa-cel). Clinical trials for both gene therapies have demonstrated high rates of sustained transfusion independence for both pediatric and adult age groups. However, despite these advances, challenges remain. Gene therapy requires myeloablative busulfan-based conditioning chemotherapy, which carries the risk of short- and long-term toxicities. Furthermore, centralized manufacturing and high treatment costs are likely to limit access to gene therapy. In this review, we discuss the available clinical trial and real-world data for beti-cel and exa-cel. We describe how gene therapy fits into the current treatment landscape and introduce areas of ongoing investigation to improve access to transformative therapy for TDT.

Plain language summary

Update on β-thalassemia treatments: Focus on Gene therapy

β-thalassemia is caused by mutations in the β-globin gene that lead to decreased hemoglobin levels (anemia). People with transfusion-dependent β-thalassemia (TDT) require lifelong blood transfusions to treat their anemia and related complications. Because chronic blood transfusion therapy causes iron buildup in the body, patients also must receive long-term daily medications to reduce the toxic effects of iron on their organs. These treatments are burdensome, costly, and can have side effects, all of which can have a negative impact on quality of life. While allogeneic stem cell transplant using stem cells from a related or unrelated donor is a curative option, this treatment is only available to patients with suitable donors and carries substantial risks of transplant-related morbidities. Gene therapy is a recently approved treatment option that offers the potential to eliminate the need for lifelong transfusions. There are two FDA approved approaches: beti-cel, which adds a healthy β-globin gene to blood stem cells, and exa-cel, which uses gene editing to produce increased amounts of fetal hemoglobin. Current data show that at least 90% of children and adults with TDT treated with either gene therapy can stop the life-long transfusions. This review discusses the clinical trial and real-world data on gene therapies for TDT, and where these fit in the overall treatment landscape. While there is much promise for gene therapy, some obstacles remain, including high cost, the need for potentially toxic chemotherapy, and still limited long-term follow-up data. Ongoing studies are aimed at making gene therapy safer, more affordable, and more widely available.

Keywords

betibeglogene autotemcel exagamglogene autotemcel gene therapy Thalassemia

Introduction

β-thalassemia is an inherited hemoglobinopathy caused by mutations in the β-globin gene (HBB), which lead to impaired production of β-globin chains. The resulting imbalance in α- and β-globin drives ineffective erythropoiesis and anemia. In patients with transfusion-dependent thalassemia (TDT), management involves lifelong chronic red cell transfusions and chelation therapy to treat acquired hemochromatosis. Thalassemia is prevalent globally, particularly in the Mediterranean, the Middle East, South Asia, and parts of Southeast Asia.¹

Despite advances in supportive care, TDT remains a lifelong blood disorder and carries considerable logistical, physical, and psychosocial burdens.² The growing population of patients living into adulthood has further highlighted the gaps in existing treatment strategies and the critical need for more effective, sustainable, and curative approaches.

Conventional therapy for TDT

Currently, standard care for TDT includes regular red blood cell transfusions, typically administered every 2–5 weeks to maintain hemoglobin levels above a threshold sufficient to suppress ineffective erythropoiesis and support normal growth and development.^3,4 Chronic transfusions inevitably lead to iron overload, so iron chelation therapy is required to control iron burden and prevent end-organ complications such as heart failure, cirrhosis, and endocrine dysfunction. Adherence to iron chelation regimens can be challenging due to side effects, treatment fatigue, and complex dosing schedules, often resulting in suboptimal control of iron overload and reduced quality of life.⁵

In recent years, pharmacologic options have emerged to reduce transfusion burden in TDT. Luspatercept is an erythroid maturation agent that has demonstrated the ability to reduce transfusion requirements in a subset of patients.^6,7 Mitapivat and etavopivat, two pyruvate kinase activators, are currently under study for their potential to improve erythropoiesis and red cell lifespan and mitigate anemia in TDT.^8,9 However, none of these agents provides a definitive cure. The ongoing unmet need for curative therapies in TDT highlights the significant potential for gene therapy to greatly impact the landscape of thalassemia treatment.

Gene therapy for β-thalassemia

Gene therapy aims to address the underlying genetic defect in β-thalassemia by raising β- or β-like globin levels to improve the α:β imbalance. Gene therapy approaches for β-thalassemia can be broadly classified into gene addition and gene editing strategies. Gene addition involves introducing a functional copy of the β-globin gene into hematopoietic stem and progenitor cells (HSPCs), the approach utilized for betibeglogene autotemcel (beti-cel). Gene editing aims to modify existing genes within HSPCs, such as to induce fetal hemoglobin (HbF) expression, as in exagamglogene autotemcel (exa-cel). Alternative gene editing approaches aim to correct the underlying mutation and are being studied in sickle cell disease.^10,11

The entire gene therapy process from cell collection to infusion involves multiple highly coordinated steps that typically span many months (Figure 1). First, eligible patients undergo mobilization and apheresis to collect autologous CD34⁺ HSPCs from the peripheral blood. Granulocyte-colony stimulating factor (G-CSF) and plerixafor are administered for stem cell mobilization prior to apheresis for stem cell collection. Once collected, cells are shipped to a manufacturing facility to perform gene modification ex vivo. Quality control is performed, and modified cells are cryopreserved and returned to the treatment center. Patients then undergo myeloablative conditioning with busulfan to create marrow space for engraftment of the modified HSPCs. Following the infusion of the gene-modified product, patients remain hospitalized and are closely monitored for neutrophil and platelet engraftment and short-term complications like infection and chemotoxicity. This personalized, resource-intensive process exemplifies the strengths and challenges of current gene therapy paradigms for TDT. In this review, we will describe the available gene therapy products and draw comparisons based on available clinical trials and real-world data. Trial data and outcomes are summarized in Table 1. We will discuss the two gene therapy products approved by the Food and Drug Administration (FDA) in the context of other available curative therapies.

Figure 1.

Timeline of the gene therapy process. The process from patient selection to receiving gene therapy requires many coordinated steps spanning several months. (1) Patients undergo informed consent with the hematology and transplant teams. (2) Patients undergo multidisciplinary evaluation to ensure clinical stability and readiness to proceed, and to develop risk mitigation strategies such as transfusion support for heavily alloimmunized patients. (3) Patients undergo hypertransfusion (target hemoglobin of ⩾11 g/dL to suppress endogenous erythropoiesis typically for at least 30 days prior to collection. (4) Patients receive filgrastim and plerixafor to mobilize CD34+ HSPCs and undergo collection via apheresis. More than one round of collection may be needed to reach the target CD34+ dose. (5) HSPCs are shipped to a manufacturing facility to undergo gene modification, testing, and release. This process may take up to 6 months. (6) Once the gene-modified product is available, patients are admitted to the hospital to receive myeloablative busulfan, followed by infusion of the modified HSPCs. Patients remain hospitalized for monitoring and management of adverse effects while awaiting engraftment, typically 4 to 7 weeks postinfusion.

Table 1.

Comparison between Beti-cel and Exa-cel trials.

Characteristic	Beti-cel^12,13	Exa-cel¹⁴
Phase III trials	HGB-207 and HGB-212	CLIMB THAL-111
Therapy type	Gene addition	Gene editing
Mechanism	Lentiviral-mediated HBB gene transfer	CRISPR-Cas9-mediated BCL11A disruption
Gene target	HBB	BCL11A erythroid specific enhancer
Number of patients (Full Analysis Set)	22 (HGB-207) + 18 (HGB-212) = 41	35
Age, years	HGB-207: Median 15 HGB-212: Median 12.5	Mean: 21.1¹⁵
Age range, years	4 to 34	12 to 35¹⁵
FDA approval ages	Pediatric and adult patients	12 years or older
Difference in potential risks	Risk of insertional oncogenesis	Off-target genome editing risk
Primary endpoint	Weighted average Hb ⩾ 9 g/dL for 12 months, starting 60 days after last transfusion
Transfusion independence (TI), %	91% (non-β⁰/β⁰)	100% (non-β⁰/β⁰-like)
	89% (β⁰/β⁰)	85% (β⁰/β⁰-like)
Duration of TI, median, months	HGB-207: 20.4	22.5
	HGB-212: 38.7
Follow-up, median (months)	HGB-207: 29.5	20.4
	HGB-212: 47.9
Duration of long-term follow-up	Up to 10 years (LTF-303)¹⁶	Up to 5 years (CLIMB-131)¹⁵
Total hemoglobin level during TI, mean, g/dL	HGB-207: 11.7	13.1
	HGB-212: 10.5
Graft failure	None	None
Neutrophil engraftment, median (range), days	HGB-207: 23 (13–32)	29 (12–56)
	HGB-212: 26 (14–39)
Platelet engraftment, median (range), days	HGB-207: 46 (20–94)	44 (20–200)
	HGB-212: 49.5 (21–64)
U.S. approval year	2022	2024

Beti-cel, the inaugural gene therapy for β-thalassemia

Beti-cel was the first gene therapy approved for β-thalassemia in the European Union in 2019 and later by the FDA in the United States in August 2022. It uses a lentiviral vector (BB305) to deliver a modified, functional HBB gene variant (β^A-T87Q) into autologous CD34+ HSPCs, to enable the production of adult hemoglobin in patients with TDT.

The clinical efficacy of beti-cel has been demonstrated across two-phase III trials and in a variety of β-thalassemia genotypes. The phase III HGB-207 trial enrolled 23 children and adults with non-β⁰/β⁰ genotypes, and transfusion independence was achieved in 20 of 22 (91%) evaluable patients, including 86% of those under 12 years old.¹² Transfusion independence was defined as maintaining a weighted average hemoglobin level of at least 9 g/dL for at least 12 months starting 60 days after the last transfusion and without additional transfusions. The median duration of transfusion independence was more than 20 months, with patients maintaining an average hemoglobin of 11.7 g/dL during this period. None of those who achieved transfusion independence required subsequent transfusions. Adverse events were related to myeloablative conditioning using busulfan, and no graft failures or secondary malignancies were observed.

Follow-up data also demonstrated improvements in markers of ineffective erythropoiesis. Among the transfusion-independent patients, the median myeloid-to-erythroid ratio improved from 1:2.4 at baseline to 1:1.2 at 24 months.¹² However, these numbers remained outside the normal range, suggesting evidence of residual ineffective erythropoiesis. Similarly, soluble transferrin receptor levels and erythropoietin decreased but did not fully normalize in the individuals who achieved transfusion independence. Trends in iron parameters, including liver iron content and myocardial iron, improved over time with iron reduction therapy among patients who had transfusion independence. Still, longer-term follow-up is needed to confirm durable improvements in iron burden. The two patients who did not achieve transfusion independence in the HGB-207 trial exhibited lower circulating vector copy numbers and had evidence of persistent ineffective erythropoiesis, with26 a higher myeloid-to-erythroid ratio and soluble transferrin receptor compared to those who stopped transfusions.¹²

Beti-cel was further evaluated in a phase III trial (HGB-212) for patients with more severe genotypes, including β⁰/β⁰.¹³ In this study, 16 of 18 (89%) patients achieved and maintained transfusion independence at a median follow-up of nearly 48 months. In follow-up, patients had robust HbA^T87Q expression and durable response. Of those who achieved transfusion independence, there was a reduction in median erythropoietin and soluble transferrin receptor levels, although this change did not reach statistical significance. Long-term follow-up from the LTF-303 study (NCT02633943), which includes patients who completed either a phase I/II or phase III beti-cel parent study, supports that transfusion independence is sustained. Of the 41 patients from the phase 3 studies, 37 (90.2%) remained transfusion-independent for up to 6 years.¹⁶ Subsequent analyses identified transduction efficiency as the strongest predictor of achieving a peripheral blood vector copy number ⩾0.75 copies/diploid genome (a surrogate for achieving transfusion independence). Drug products with fewer than approximately 60% lentiviral vector-positive cells predicted failure to achieve transfusion independence.¹⁷ Iron burden improved among the 37 participants who achieved transfusion independence, with 22 (59%) able to discontinue iron reduction therapy with liver iron concentration below 5 mg/g dw.¹⁶

More recently, the first real-world cohort of patients with non-β⁰/β⁰ TDT treated with beti-cel was reported from Germany.¹⁸ Of the eight described patients with non-β⁰/β⁰ genotypes, all were able to discontinue regular transfusions within 8–59 days following infusion, and all remained transfusion-free at last follow-up (median follow-up 541.5 days, range 365–734). Pharmacodynamic evaluation demonstrated stable and durable HbA^T87Q up to month 24, and the safety profile was consistent with clinical trials. Notably, follow-up quality of life assessments demonstrated mixed results; while 6 of 8 patients resumed school or work a year after undergoing treatment with beti-cel, one patient newly developed a fatigue syndrome, and another patient developed depression and panic attacks. One patient also developed asymptomatic polycythemia, with a peak hemoglobin of 19.3 g/dL and no identifiable molecular driver on evaluation. This patient did not receive specific treatment for polycythemia and continued to be stable and asymptomatic at over 2 years of follow-up.

As part of the regulatory evaluation of beti-cel for TDT, the Food and Drug Administration (FDA) conducted an extensive review of preclinical and clinical integration site data. Insertional oncogenesis was considered unlikely, with no observed clonal dominance or enrichment near known oncogenes in integration site analyses.¹⁹ A 15-year postmarketing study was mandated to monitor for delayed adverse events, including clonal expansion and hematologic malignancies, consistent with standard gene therapy safety protocols.

Building upon the foundation of gene therapy established by beti-cel, new therapeutic approaches have emerged, most notably the application of gene editing technologies.

Exa-cel, a novel gene editing curative approach to β-thalassemia

BCL11A is a transcriptional regulator and repressor of the human γ-globin gene, identified through genome-wide association studies as a key regulator of fetal hemoglobin (HbF) expression.^20–22 Knockdown of BCL11A in human erythroid precursor cells resulted in increased HbF. In mouse models of sickle cell disease, BCL11A knockdown effectively ameliorated symptoms of sickle cell disease and reactivated γ-globin.^23,24 Furthermore, the protective effect of fetal hemoglobin is clinically evident, as anemia and symptoms of thalassemia and sickle cell disease do not typically develop before 3 months of age, when HbF levels are physiologically high. This rationale led to the development of exagamglogene autotemcel (exa-cel), which uses CRISPR-Cas9, a ribonucleic acid-guided deoxyribonucleic acid-targeting platform adapted from bacterial defense systems, to disrupt the erythroid-specific enhancer of BCL11A.²⁵ This leads to the reactivation of endogenous HbF, compensating for deficient or absent adult hemoglobin in TDT (Figure 2). Exa-cel was approved by the FDA in January 2024 for the treatment of individuals with TDT 12 years and older. An ongoing study is assessing safety and efficacy of exa-cel in younger children ages 2 to 11 (NCT05356195).

Figure 2.

Mechanism of action of exagamglogene autotemcel. Autologous CD34+ HSPCs are collected and undergo ex vivo CRISPR-Cas9 gene editing. The editing targets and disrupts the erythroid-specific enhancer located in intron 2 of BCL11A gene. BCL11A is a key transcriptional regulator responsible for silencing fetal hemoglobin (HbF) expression in a physiologic phenomenon commonly known as the hemoglobin switch. Disruption of its enhancer selectively downregulates BCL11A expression in erythroid cells, reactivating HbF production by disinhibiting the expression of γ-globin. This compensates for deficient or absent β-globin production to reduce or eliminate transfusion requirements in patients with transfusion-dependent β-thalassemia.

The phase III CLIMB THAL-111 trial enrolled a total of 52 patients, ages 12 to 35 years, with TDT across multiple genotypes (β⁰/β⁰, β⁰/β⁰-like, or non-β⁰/β⁰-like).¹⁴ Of the 35 evaluable patients, transfusion independence was achieved in 32 individuals (91%). The median duration of follow-up was 20.4 months, and the mean duration of transfusion independence was 22.5 months. All evaluable patients achieved successful editing at the BCL11A enhancer site, with high levels of editing stably detectable in the peripheral blood and bone marrow throughout the follow-up period. As in the beti-cel trials, the most common adverse events were attributed to busulfan myeloablative conditioning, with the most common grade 3 or grade 4 events being febrile neutropenia and stomatitis. No patients experienced graft failure or malignancy. In a study of CD34+ HSPCs from eight healthy donors and three donors with TDT, no off-target editing was detected with the CRISPR-Cas9 gene editing technique.²⁶

Biomarker follow-up indicated potential improvement of the underlying ineffective erythropoiesis associated with TDT, although values remained outside the normal range. The myeloid-to-erythroid ratio in the bone marrow increased over 24 months from 0.62 ± 0.45 at baseline to 0.81 ± 0.42 in 15 patients with available data (normal ratio 1.2–5), signaling an improvement in erythropoiesis. Additional laboratory values related to erythropoiesis (reticulocyte count and nucleated red blood cell count) remained stable, comparing baseline to 24-month follow-up.

Data from the CLIMB-131 long-term follow-up study demonstrate the durability of clinical benefit in patients with TDT who received exa-cel in the CLIMB-111 core study. Among 52 evaluable participants, 49 (94.2%) achieved transfusion independence for at least 12 consecutive months; they maintained transfusion independence for the entire follow-up period (mean 32.4 months, range 14.3–60.8 months). Total hemoglobin and HbF production remained stable, with pancellular HbF distribution. Edited BCL11A alleles were stable in the marrow and peripheral blood. With iron reduction therapy, iron burden improved with 26 (46.4%) participants discontinuing iron removal therapy.¹⁵

Given the use of CRISPR/Cas9 gene editing to induce double-strand breaks at the BCL11A erythroid enhancer, exa-cel raises theoretical concerns regarding off-target effects leading to clonal hematopoiesis and oncogenic transformation. However, to date, no cases of malignancy or clonal expansion have been observed in clinical trials or preclinical models. Nonclinical studies demonstrated no evidence of tumorigenicity, chromosomal aberrations, or off-target editing in edited CD34+ HSPCs. Similar to the regulatory review for beti-cel, the FDA has required a 15-year post-marketing study to monitor for hematologic malignancies and long-term safety.

Contextualizing gene therapy in the landscape of curative approaches

Allogeneic hematopoietic stem cell transplantation (HSCT) has been the cornerstone of curative therapy for TDT, and the most favorable outcomes are typically achieved in children who receive transplants from HLA-matched sibling donors. In this context, thalassemia-free survival (TFS) rates exceed 80%, with children younger than 14 years achieving higher TFS rates of 83%–93%.^27,28 However, the limited availability of suitable donors restricts the widespread use of allogeneic HSCT for TDT. Alternative approaches, such as unrelated, haploidentical, or mismatched transplants, carry significantly higher risks of graft failure, graft-versus-host disease (GVHD), and transplant-related mortality, and are generally considered inferior.²⁹ There are promising data emerging from select centers exploring unrelated donor stem cell transplantation using partial T-cell depletion in pediatric patients with hemoglobinopathies. This approach has been shown to reduce the potential for GVHD and appears safe and effective for patients with TDT who have at least 9/10 HLA-matched donors.^30–32 These findings support the consideration of this approach at experienced centers within the framework of a research trial. In parallel, ongoing research is evaluating allogeneic transplantation for thalassemia using haploidentical donors, with efforts focusing on optimizing conditioning regimens, T-cell depletion methods, and posttransplant GVHD prophylaxis, to maximize TFS while minimizing transplant-related morbidities.³³

The introduction of gene therapy has already begun to redefine the therapeutic landscape for TDT. Gene therapy is an autologous intervention that obviates the need for a matched donor, potentially expanding access to transformative therapy to a broader segment of the TDT community. Gene therapy also eliminates the risk of GVHD, avoiding the need for posttransplant immunosuppression, thereby reducing the risk of infection compared with allogeneic HSCT. Because of these important differences, gene therapy offers a chance at a cure while bypassing the acute and chronic toxicities typically associated with allogeneic HSCT. However, platelet engraftment following gene therapy tends to occur later than with allogeneic HSCT, which may prolong the period of transfusion support and bleeding risk during the initial recovery phase.^{12,13,14,34–36} This delay may be particularly relevant in patients with a high panel-reactive antibody percentage, who may face greater challenges with platelet refractoriness or alloimmunization during the engraftment window.³⁷ Furthermore, as with allogeneic HSCT, the conditioning chemotherapy required for gene therapy carries a long-term risk of infertility, and requires early fertility counseling and consideration of fertility preservation.³⁸

Despite these advances, allogeneic HSCT remains the standard of care for young patients with an available HLA-matched sibling donor, given the high rates of associated overall survival and TFS. However, an apparent age-related decline in outcomes has been observed. In patients undergoing allogeneic HSCT at or after age 18, overall survival drops to approximately 80%, and TFS to 76%, reflecting a narrower therapeutic window for allogeneic transplants.²⁷ In this context, gene therapy represents a particularly compelling option for adolescents and adults with TDT, for whom allogeneic HSCT carries a higher risk. Clinical trial data consistently show that gene therapy can achieve transfusion independence in adult patients, with efficacy that rivals—if not exceeds—the outcomes historically achieved with allogeneic HSCT in younger cohorts. Notably, unlike in allogeneic HSCTs, gene therapy outcomes appear similar in pediatric and adult patients. However, the available trial data remain limited to individuals under age 35, and it is unknown how outcomes may be modified by age- and iron-related comorbidities in older adults. Further study is needed in older adults to better define the safety and efficacy of gene therapy beyond the currently studied age range.

While direct head-to-head comparisons between exa-cel and beti-cel are lacking, available data indicate that rates of transfusion independence appear similar across the two therapies. Trial results suggest that neutrophil engraftment may take several days longer with exa-cel compared to beti-cel. However, this difference does not appear to impact the rate of adverse events or overall outcomes. In clinical practice, the choice between these products will depend on shared decision-making, manufacturing availability and timelines, and cost or insurance coverage considerations. Although exa-cel leads to chronically elevated HbF levels, data from individuals with hereditary persistence of fetal hemoglobin suggest that this is well-tolerated, and any potential associations with outcomes such as lower fetal birthweight remain speculative.^39–42 Similarly, while insertional oncogenesis or off-target effects are theoretical risks (particularly with beti-cel’s integrating lentiviral vector), no such events have been reported in clinical trials for either product. As such, these safety considerations are unlikely to be the primary drivers of clinical decision-making.

Current challenges to gene therapy

Despite the transformative potential of gene therapy for TDT, numerous challenges hinder its widespread adoption and equitable access. First, the global epidemiology of thalassemia reveals a stark mismatch between disease burden and therapeutic availability: the highest prevalence of TDT is concentrated in low- and middle-income countries where access to gene therapy is considerably lower given limited infrastructure, healthcare capacity, and prohibitive cost.^43,44

Even in high-resource settings, substantial logistical and socioeconomic barriers may constrain access to gene therapy. The treatment process—from stem cell mobilization and apheresis to conditioning, hospital admission, and cell recovery—spans months, imposing a significant time and financial burden on patients and families. For many, the need to take prolonged time off work or arrange extended caregiving support makes this option impractical. Moreover, the upfront costs of gene therapy, which run in the millions, present a challenge to payers and healthcare systems. Alternative payment models are now being explored by payers and manufacturers, including milestone-based rebates tied to predefined clinical outcomes, outcomes-based warranties, and performance-based installment payments.⁴⁵ These approaches are intended to address uncertainties related to the durability of gene therapy and to better align costs with clinical benefit in order to mitigate payer risk. However, most of these arrangements remain in early stages of implementation, and their design and availability vary considerably across geographic regions, health systems, and payers.

Individual perceptions of risks and benefits further shape patient uptake. Given the novelty of gene therapy and the relatively short duration of posttreatment follow-up, uncertainty remains around long-term safety, efficacy, and durability. A recent discrete choice experiment in sickle cell disease (SCD) found that patient willingness to pursue gene therapy varied depending on disease severity and the perceived risk of treatment-related mortality.⁴⁶ Decision factors may be significantly different for individuals with TDT. In contrast to those with SCD in whom unpredictable and life-threatening complications may compel patients toward gene therapy, patients with TDT are often clinically stable on chronic transfusion and iron chelation regimens. In this context, the threshold to pursue gene therapy may be higher in TDT, especially in those who do not have daily symptoms impacting quality of life. Safety concerns, particularly the theoretical risk of secondary malignancies from myeloablative conditioning with busulfan or insertional oncogenesis or off-target effects, also remain a source of hesitation.^47,48 Although no cases of hematologic malignancy have been reported in gene therapy recipients with TDT to date, the long-term oncogenic risk remains undefined.

The risk of treatment-related infertility is also a particularly relevant consideration, especially because many patients considering gene therapy are young and have not yet started families. Busulfan is gonadotoxic and can permanently impair fertility, and fertility preservation options such as oocyte or embryo cryopreservation, sperm banking, or ovarian/testicular cryopreservation in prepubertal children are logistically complex and costly.⁴⁹ In addition, testicular tissue cryopreservation remains experimental as the success at achieving future pregnancies has not been established. Furthermore, insurance coverage for fertility preservation is inconsistent, and out-of-pocket costs to patients/families can be substantial. Together, these burdens represent additional challenges for patients and their families and may further discourage the decision to proceed with gene therapy.

Finally, while gene therapy is often described as curative, this characterization may oversimplify the complex physiology of thalassemia. One ongoing question is whether gene therapy fully addresses ineffective erythropoiesis, a hallmark of β-thalassemia. If patients achieve transfusion independence but have persistent ineffective erythropoiesis, they may continue to be at risk for long-term complications such as extramedullary hematopoiesis, low bone mass, and iron overload. In this theoretical situation, they may transition into a non-TDT phenotype, complicating claims of cure.⁵⁰ Further study is needed to characterize the long-term durability of transfusion independence, the trajectory of biomarkers and clinical indicators associated with ineffective erythropoiesis, and the potential for long-term complications, to more accurately inform patients of risks and benefits associated with gene therapy.

Future directions

Despite these challenges, the introduction and approval of gene therapy for TDT represents an important step toward improving efficacy and safety in future iterations. Multiple strategies are being investigated with the goal of improving the likelihood of achieving transfusion independence and enhancing hemoglobin correction beyond the minimal threshold needed to discontinue transfusions. Given the high vector copy numbers required with current lentiviral-based approaches, novel lentiviral vectors are being developed to improve gene delivery and enhance phenotypic correction.^51–53 In parallel, alternative genetic modification techniques such as base editing are being explored to correct specific β-thalassemia mutations directly.^54–56

Beyond refining the gene delivery pipeline, efforts are also underway to reduce or eliminate the need for myeloablative chemotherapy, which currently represents a significant barrier to the broad acceptance of gene therapy, given concerns over acute chemotherapy-related toxicities and longer-term complications such as busulfan-related infertility and secondary malignancies. A phase 1 study evaluating reduced-intensity busulfan conditioning prior to infusion of TNS9.3.55 lentiviral-modified autologous CD34+ cells reduced transfusion needs in two of four patients but did not achieve transfusion independence.⁵⁷ Other approaches, including antibody-based conditioning regimens, are in preclinical development but have not yet advanced to human trials in thalassemia.⁵⁸ Furthermore, in vivo gene therapy strategies are being explored as a means of overcoming the need for stem cell collection or cytotoxic conditioning. These approaches are early in development but may dramatically reduce treatment-related toxicities and expand access to gene therapy.⁵⁹

Ultimately, future directions in gene therapy for TDT must focus on improving efficacy and safety, expanding access, and reducing the overall burden of therapy for patients. Future research efforts to improve vector design, conditioning regimens, and gene therapy manufacturing processes will be essential to more fully realize the potential of gene therapy as a broadly accessible option for individuals with TDT.

Limitations

This review has several important limitations. First, this is not a formal systematic review; while we attempted to be comprehensive in our literature search, relevant studies may have been missed. Second, the absence of head-to-head comparisons between available gene therapies or comparing gene therapy to allogeneic transplant or with other treatment modalities limits our ability to directly compare studies. Finally, we acknowledge that the landscape of genetic therapies and of allogeneic transplants is rapidly evolving, and new evidence may soon update the findings presented in this review. We therefore caution the interpretation of this literature review as an exhaustive summary of the field.

Conclusion

Beti-cel and exa-cel represent significant additions to the therapeutic options available to reduce the burdens of chronic transfusions and iron chelation in TDT. While trial data are promising, demonstrating high rates of transfusion independence and durable responses, the long-term safety and efficacy of gene therapy are still unknown, and global accessibility remains a limitation. Broader adoption will require reducing barriers to access and deepening our understanding of long-term outcomes.

Footnotes

Acknowledgements

We are grateful to Vivian Jou, PhD, for contributing the figures included in this review article.

Declarations

ORCID iDs

Aaron N. Cheng

Janet L. Kwiatkowski

References

Musallam

Lombard

Kistler

, et al. Epidemiology of clinically significant forms of alpha- and β-thalassemia: a global map of evidence and gaps. American J Hematol 2023; 98(9): 1436–1451.

Paramore

Levine

Bagshaw

, et al. Patient- and caregiver-reported burden of transfusion-dependent β-thalassemia measured using a digital application. Patient 2021; 14(2): 197–208.

Musallam

Cappellini

Porter

, et al. TIF guidelines for the management of transfusion-dependent β-thalassemia. Hemasphere 2025; 9(3): e70095.

Musallam

Barella

Origa

, et al. Pretransfusion hemoglobin level and mortality in adults with transfusion-dependent β-thalassemia. Blood 2024; 143(10): 930–932.

Lee

Mohd Tahir

Chun

, et al. The impact of chelation compliance in health outcome and health related quality of life in thalassaemia patients: a systematic review. Health Qual Life Outcomes 2024; 22(1): 14.

Cappellini

Viprakasit

Taher

, et al. A Phase 3 Trial of luspatercept in patients with transfusion-dependent β-thalassemia. N Engl J Med 2020; 382(13): 1219–1231.

Cappellini

Viprakasit

Georgiev

, et al. Long-term efficacy and safety of luspatercept for the treatment of anaemia in patients with transfusion-dependent β-thalassaemia (BELIEVE): final results from a phase 3 randomised trial. Lancet Haematol 2025; 12(3): e180–e189.

Cappellini

Sheth

Taher

, et al. ENERGIZE-T: a global, phase 3, double-blind, randomized, placebo-controlled study of Mitapivat in adults with transfusion-dependent alpha- or beta-thalassemia. Blood. 2024; 144(Suppl. 1): 409–409.

Kuo

KHM

. Pyruvate kinase activators: targeting red cell metabolism in thalassemia. Hematology Am Soc Hematol Educ Program 2023; 2023(1): 114–120.

10.

Newby

Yen

Woodard

, et al. Base editing of haematopoietic stem cells rescues sickle cell disease in mice. Nature 2021; 595(7866): 295–302.

11.

Lattanzi

Camarena

Lahiri

, et al. Development of β-globin gene correction in human hematopoietic stem cells as a potential durable treatment for sickle cell disease. Sci Transl Med 2021; 13(598): eabf2444.

12.

Locatelli

Thompson

Kwiatkowski

, et al. Betibeglogene autotemcel gene therapy for non–β⁰ /β⁰ genotype β-thalassemia. N Engl J Med 2022; 386(5): 415-427.

13.

Kwiatkowski

Walters

Hongeng

, et al. Betibeglogene autotemcel gene therapy in patients with transfusion-dependent, severe genotype β-thalassaemia (HGB-212): a non-randomised, multicentre, single-arm, open-label, single-dose, phase 3 trial. Lancet 2024; 404(10468): 2175–2186.

14.

Locatelli

Lang

Wall

, et al. Exagamglogene Autotemcel for Transfusion-Dependent β-Thalassemia. N Engl J Med 2024; 390(18): 1663–1676.

15.

Locatelli

Lang

Meisel

, et al. Durable clinical benefits with exagamglogene autotemcel for transfusion-dependent β-thalassemia. Blood 2024; 144(Suppl. 1): 512–512.

16.

Thompson

Kwiatkowski

Schneiderman

, et al. Betibeglogene Autotemcel (beti-cel) gene addition therapy results in durable hemoglobin a production with up to 10 years of follow-up with transfusion-dependent β-thalassemia. Blood 2024; 144(Suppl. 1): 2194–2194.

17.

Whitney

Shestopalov

Fincker

, et al. Drug product attributes predict clinical efficacy in betibeglogene autotemcel gene therapy for β-thalassemia. Mol Ther Methods Clin Devel 2023; 31: 101155.

18.

Mirza

Ritsert

Tao

, et al. Gene therapy in transfusion-dependent non-β0/β0 genotype β-thalassemia: first real-world experience of beti-cel. Blood Adv 2025; 9(1): 29–38.

19.

FDA Zynteglo Approval history, letters, reviews and related documents. FDA. https://www.fda.gov/vaccines-blood-biologics/zynteglo (2025, accessed 29 July 2025).

20.

Lettre

Sankaran

Bezerra

MAC

, et al. DNA polymorphisms at the BCL11A, HBS1L-MYB, and β-globin loci associate with fetal hemoglobin levels and pain crises in sickle cell disease. Proc Natl Acad Sci USA 2008; 105(33): 11869–11874.

21.

Uda

Galanello

Sanna

, et al. Genome-wide association study shows BCL11A associated with persistent fetal hemoglobin and amelioration of the phenotype of β-thalassemia. Proc Natl Acad Sci USA 2008; 105(5): 1620–1625.

22.

Menzel

Garner

Gut

, et al. A QTL influencing F cell production maps to a gene encoding a zinc-finger protein on chromosome 2p15. Nat Genet 2007; 39(10): 1197–1199.

23.

Peng

Sankaran

, et al. Correction of sickle cell disease in adult mice by interference with fetal hemoglobin silencing. Science 2011; 334(6058): 993–996.

24.

Sankaran

Menne

, et al. Human fetal hemoglobin expression is regulated by the developmental stage-specific repressor BCL11A. Science 2008; 322(5909): 1839–1842.

25.

Jinek

Chylinski

Fonfara

, et al. A Programmable dual-RNA–guided DNA endonuclease in adaptive bacterial immunity. Science 2012; 337(6096): 816–821.

26.

Yen

Zappala

Fine

Majarian

, et al. Specificity of CRISPR-Cas9 editing in exagamglogene autotemcel. N Engl J Med 2024; 390(18): 1723–1725.

27.

Baronciani

Angelucci

Potschger

, et al. Hemopoietic stem cell transplantation in thalassemia: a report from the European Society for Blood and Bone Marrow Transplantation Hemoglobinopathy Registry, 2000–2010. Bone Marrow Transplant 2016; 51(4): 536–541.

28.

Locatelli

Kabbara

Ruggeri

, et al. Outcome of patients with hemoglobinopathies given either cord blood or bone marrow transplantation from an HLA-identical sibling. Blood 2013; 122(6): 1072–1078.

29.

Mathews

Kim

, et al. Related and unrelated donor transplantation for β-thalassemia major: results of an international survey. Blood Adv 2019; 3(17): 2562–2570.

30.

Gaziev

Isgrò

Sodani

, et al. Haploidentical HSCT for hemoglobinopathies: improved outcomes with TCRαβ+/CD19+-depleted grafts. Blood Adv 2018; 2(3): 263–270.

31.

Merli

Pagliara

Galaverna

, et al. TCRαβ/CD19 depleted HSCT from an HLA-haploidentical relative to treat children with different nonmalignant disorders. Blood Adv 2022; 6(1): 281–292.

32.

Gibson

Elgarten

Oved

, et al. Outcomes of unrelated donor stem cell transplantation with partial T cell depletion for pediatric patients with hemoglobinopathies. Transpl Cell Ther 2025; 31(10): 828.e1–828.e14.

33.

Xiao

Huang

Lai

, et al. Haploidentical hematopoietic stem cell transplantation in pediatric transfusion-dependent thalassemia: a systematic review and meta-analysis. Transpl Cell Ther 2025; 31(2): 101.e1–101.e12.

34.

Bernardo

Piras

Vacca

, et al. Allogeneic hematopoietic stem cell transplantation in thalassemia major: results of a reduced-toxicity conditioning regimen based on the use of treosulfan. Blood 2012; 120(2): 473–476.

35.

Sellathamby

Lakshmi

Busson

, et al. Polymorphisms in the immunoregulatory genes are associated with hematopoietic recovery and increased susceptibility to bacterial infections in patients with thalassaemia major undergoing matched related hematopoietic stem cell transplantation. Biol Blood Marrow Transpl 2012; 18(8): 1219–1226.

36.

Sun

Wang

Chen

, et al. Unrelated donor peripheral blood stem cell transplantation for patients with β-thalassemia major based on a novel conditioning regimen. Biol Blood Marrow Transpl 2019; 25(8): 1592–1596.

37.

Gibson

Khandros

Elgarten

, et al. Impact of HLA alloimmunization in gene-modified autologous stem cell transplant for transfusion-dependent thalassemia. Blood 2025; 145(22): 2666–2670.

38.

Hmaidan

Van Heertum

Bhatia

, et al. Safety and feasibility of fertility preservation and fertility outcomes in patients with sickle cell disease and transfusion-dependent beta thalassemia undergoing gene therapy. Transpl Cell Ther 2025; 31(11): 934.e1–934.e8.

39.

Forget

BG.

Molecular basis of hereditary persistence of fetal hemoglobin. Ann N York Acad Sci 1998; 850(1): 38–44.

40.

Thein

Menzel

Lathrop

, et al. Control of fetal hemoglobin: new insights emerging from genomics and clinical implications. Human Mol Genet 2009; 18(R2): R216–R223.

41.

Steinberg

MH.

Fetal hemoglobin in sickle hemoglobinopathies: High HbF genotypes and phenotypes. JCM 2020; 9(11): 3782.

42.

Murji

Sobel

Hasan

, et al. Pregnancy outcomes in women with elevated levels of fetal hemoglobin. J Matern Fetal Neonatal Med 2012; 25(2): 125–129.

43.

Kattamis

Forni

Aydinok

, et al. Changing patterns in the epidemiology of β-thalassemia. Eur J Haematol 2020; 105(6): 692–703.

44.

Adair

Androski

Bayigga

, et al. Towards access for all: 1st working group report for the global gene therapy initiative(Ggti). Gene Ther 2023; 30(3–4): 216–221.

45.

Phares

Trusheim

Emond

, et al. Managing the challenges of paying for gene therapy: strategies for market action and policy reform in the United States. J Comp Eff Res 2024; 13(12): e240118.

46.

Gonzalez Sepulveda

Yang

Reed

, et al. Preferences for potential benefits and risks for gene therapy in the treatment of sickle cell disease. Blood Adv 2023; 7(23): 7371–7381.

47.

Quarmyne

Ross

Sinha

, et al. Decision-making about gene therapy in transfusion dependent thalassemia. BMC Pediatr 2022; 22(1): 536.

48.

Thuret

Ruggeri

Angelucci

, et al. Hurdles to the adoption of gene therapy as a curative option for transfusion-dependent thalassemia. Stem Cells Transl Med 2022; 11(4): 407–414.

49.

Haering

Coyne

Daunov

, et al. Ovarian tissue cryopreservation for fertility preservation in patients with hemoglobin disorders: a comprehensive review. J Clin Med 2024; 13(13): 3631.

50.

Soni

Gene therapies for transfusion dependent β-thalassemia: current status and critical criteria for success. Am J Hematol 2020; 95(9): 1099–1112.

51.

Ling

Wang

, et al. Modified lentiviral globin gene therapy for pediatric β0/β0 transfusion-dependent β-thalassemia: a single-center, single-arm pilot trial. Cell Stem Cell 2024; 31(7): 961–973.e8.

52.

Koniali

Flouri

Kostopoulou

, et al. Evaluation of mono- and bi-functional GLOBE-based vectors for therapy of β-thalassemia by HBBAS3 gene addition and mutation-specific RNA interference. Cells 2023; 12(24): 2848.

53.

Breda

Ghiaccio

Tanaka

, et al. Lentiviral vector ALS20 yields high hemoglobin levels with low genomic integrations for treatment of β-globinopathies. Mol Ther 2021; 29(4): 1625–1638.

54.

Badat

Ejaz

Hua

, et al. Direct correction of haemoglobin E β-thalassaemia using base editors. Nat Commun 2023; 14(1): 2238.

55.

Hardouin

Antoniou

Martinucci

, et al. Adenine base editor-mediated correction of the common and severe IVS1-110 (G>A) β-thalassemia mutation. Blood 2023; 141(10): 1169–1179.

56.

Prasad

Devaraju

George

, et al. Precise correction of a spectrum of β-thalassemia mutations in coding and non-coding regions by base editors. Molecular Therapy - Nucleic Acids 2024; 35(2): 102205.

57.

Boulad

Maggio

Wang

, et al. Lentiviral globin gene therapy with reduced-intensity conditioning in adults with β-thalassemia: a phase 1 trial. Nat Med 2022; 28(1): 63–70.

58.

Uchida

Stasula

Demirci

, et al. Fertility-preserving myeloablative conditioning using single-dose CD117 antibody-drug conjugate in a rhesus gene therapy model. Nat Commun 2023; 14(1): 6291.

59.

Breda

Papp

Triebwasser

, et al. In vivo hematopoietic stem cell modification by mRNA delivery. Science 2023; 381(6656): 436–443.