Gene therapy for sickle cell disease: Practice recommendations from the American Society for Transplantation and Cellular Therapy and the International Society for Cell & Gene Therapy

Abstract

Gene therapy has emerged as a transformative treatment option for individuals with sickle cell disease (SCD), with recent regulatory approvals marking a pivotal shift in clinical care. However, the complexity of patient selection, stem cell mobilization, manufacturing, conditioning, and long-term follow-up underscores the need for standardized, evidence-informed guidance. This consensus document, derived from early clinical experience, and developed by the American Society for Transplantation and Cellular Therapy and the International Society for Cell & Gene Therapy, provides practical recommendations for the clinical implementation of ex vivo gene therapies for SCD. Key areas addressed include eligibility assessment, comparative considerations with allogeneic hematopoietic cell transplantation, mobilization and apheresis strategies, conditioning with myeloablative chemotherapy, fertility preservation, psychosocial care, manufacturing quality attributes, and lifelong surveillance for late effects. Collectively, these recommendations aim to harmonize clinical practice, support shared decision-making, and promote safe, equitable, and durable delivery of gene therapy for individuals with SCD.

Introduction

Sickle cell disease (SCD) is a significant global health concern affecting approximately 7.7 million individuals worldwide. Despite advances in understanding the pathophysiology of SCD and the availability of new disease-modifying drugs, many patients still experience poor disease control. Socioeconomic and logistical factors have resulted in millions of affected persons continuing to lack access to disease-modifying therapies, and these factors also limit access to curative treatments. In 2023, the U.S. Food and Drug Administration (FDA) approved the first two gene therapies for SCD, marking a transformative milestone in the management of the disease. These therapies, based on CRISPR-Cas9 gene editing or lentiviral gene addition, potentially offer a one-time, durable treatment for individuals with severe SCD. However, several aspects of the management of treatment for patients with SCD, including long-term follow-up, remain to be delineated.

Given the complexity and evolving understanding of gene therapy, and the use of high dose chemotherapy conditioning prior to infusion of autologous genetically modified hematopoietic stem cells, there is an urgent and ongoing need for clear, evidence-driven clinical guidance on its use. This article was developed by an international panel of experts with multidisciplinary representation in response to the growing demand for standardized education, harmonized clinical pathways, and cross-specialization collaboration in the application of gene therapy to SCD. The article aims to provide a framework to support healthcare professionals in navigating the gene therapy journey, from patient selection to procedural considerations, and eventually to specific SCD outcomes and late effects assessment. By promoting consistency, equity, and informed/shared decision-making, this guideline aims to maximize the impact of gene therapy and improve care for individuals living with SCD worldwide.

What evidence supports the approval of gene therapies for SCD?

Two FDA-approved gene therapies are commercially available for individuals with severe SCD. The FDA approval of exagamglogene autotemcel (exa-cel; Casgevy ^®) was based on interim efficacy and safety data from the CLIMB-121 trial, with a data cutoff of June 2023.¹ Exa-cel reactivates fetal hemoglobin synthesis in adult red blood cells (RBCs) via ex vivo CRISPR-Cas9 gene editing targeting the erythroid-specific enhancer region of the BCL11A gene.¹ Early and sustained increases in total and fetal hemoglobin were observed in patients at a median of 20 days after infusion.¹ Forty-four patients received exa-cel, with a median follow-up of 19.3 months.¹ All patients achieved engraftment of neutrophils and platelets, and the treatment safety profile was consistent with that of myeloablative busulfan conditioning and autologous hematopoietic cell transplantation (HCT).¹ The efficacy data were derived from 30 evaluable patients who were followed for at least 16 months after exa-cel infusion. Among these patients, for at least 12 consecutive months, 29 (97%) were free of vaso-occlusive crises (VOCs), and all 30 were free from hospitalization for VOCs.¹ No secondary malignancies have been observed in any of the study participants so far.

Data supporting the FDA approval of lovotibeglogene autotemcel (lovo-cel, Lyfgenia ^®) were generated from 36 patients with SCD who were followed for a median of 38 months after infusion.^2,3 Lovo-cel uses the ex vivo insertion of the BB305 lentiviral vector (LVV) into hematopoietic stem cells. This vector encodes a modified β-globin gene, resulting in the production of a form of HbA with anti-sickling properties.² Of the 32 evaluable patients, 28 (88%) experienced no vaso-occlusive events (VOEs) and 30 (94%) experienced no severe VOEs at 6–18 months after treatment. Acute myeloid leukemia developed in two study participants who had received an earlier version of this therapy that was manufactured using an older process.^3,4 An investigation of causality indicated that the leukemia was unlikely to be related to the vector insertion.^4,5 No secondary malignancies have been reported thus far in patients receiving the FDA-approved treatment.

Additional gene therapy approaches, including base editing of the γ-globin promoter⁶ and using an LVV to knock down BCL11A,⁷ are being evaluated in ongoing late-stage clinical trials. When these therapies receive regulatory approval, they may offer additional treatment options for individuals with SCD.

Who should receive gene therapy?

Patient selection for gene therapy should consider not only biological eligibility but also individual goals, social context, and the feasibility of safely completing the entire treatment course, from stem cell mobilization through long-term follow-up. The following are important patient selection considerations:

• Sickle cell genotypes: Although the FDA labels for these two therapies do not specify any genotypes or restrictions, all gene therapy recipients enrolled on the various clinical trials so far have had either HbSS or HbS β⁰-thalassemia. Additional research should be conducted in people with compound heterozygous genotypes (e.g. HbSC, HbSD etc.) before these therapies are used in a commercial setting.

• α-Thalassemia status: Altered α-globin production typically causes no significant symptoms, but the imbalance between α-globin and β-globin production within RBCs may be more pronounced in individuals receiving gene addition therapies when α-thalassemia trait is present. This may cause precipitation of excess β-globin–like chains, resulting in ineffective erythropoiesis and erythroid dysplasia.⁸ Hence, individuals with two or more α-globin deletions should not receive β-globin gene addition therapy (including lovo-cel), and exa-cel should be used with caution as data are limited.

• Age: Participants in gene therapy trials have ranged in age from 12 to 45 years. Until safety and efficacy data are available for patients younger than 12 years or older than 45 years, some programs may be reluctant to consider such candidates and insurance carriers may also deny coverage.

• Frequency of hospital visits: The inclusion criteria for the clinical trials of the approved gene therapy products included the patient having had at least two hospital encounters for VOEs requiring opioids in each of the 2 years preceding gene therapy or having had four VOEs in the 24 months before enrollment despite being on the maximum tolerated dose of hydroxyurea. Patients with very high acute care admission rates were excluded from some trials.⁸ Patients with chronic pain can experience pain resolution after undergoing gene therapy, but this may take up to 1–2 years.^9,10 These factors should be considered and discussed with prospective patients.

• Complications and co-morbidities: Adequate organ function is essential for receiving any form of ex vivo gene therapy that requires myeloablative conditioning. A history of stroke or abnormal transcranial Doppler imaging were exclusion criteria for clinical trials of the currently approved gene therapies. Preliminary data suggests that gene therapy may improve cerebral hemodynamics in individuals with SCD.¹¹ Cancer and active viral or fungal infections are also exclusionary. Individuals with iron overload, particularly when associated with liver fibrosis, are at increased risk of busulfan toxicity and the development of hepatic veno-occlusive disease/sinusoidal obstruction syndrome (VOD/SOS). Caution should therefore be exercised when selecting patients with significant iron overload, and appropriate iron chelation therapy should be implemented prior to conditioning. A bone marrow assessment is also recommended prior to collection of hematopoietic stem cells for gene therapy manufacturing to rule out any underlying bone marrow pathology. However, it is unclear at this point whether next generation sequencing based assays for assessing clonal hematopoiesis should be used to bone marrow assessment prior to gene therapy.

Given these factors, gene therapy candidacy is best determined through a multidisciplinary evaluation with explicit shared decision-making and documentation of patient preferences.

What are the pros and cons of the various treatment options with curative intent?

Current therapies for SCD with curative intent include allogeneic-HCT (allo-HCT) and the two approved gene therapies.^1,2 Allo-HCT has been limited in its application over the years because of the restricted donor pool, the risk of graft-versus-host disease (GvHD), infertility concerns, and the high rates of toxicity, particularly in individuals with end-organ disease. Recent advances in allo-HCT have addressed many of these obstacles and have enabled many more eligible individuals with SCD to undergo allo-HCT from a suitable matched or mismatched donor.^12–16 However, the risk of infertility may not be completely alleviated with the newer reduced-intensity conditioning (RIC) regimens.¹⁷ Important concerns when considering allo-HCT are the risks of GvHD and graft rejection or failure; conditioning regimen intensity must be adapted accordingly. Although no graft failure has been observed after either of the two approved gene therapies, long-term durability data are lacking.

The main advantages of gene therapy are it does not require a suitable allogeneic donor, there is no risk of graft-related immunological complications, and there is no need for post-HCT immunosuppression. Although the median follow-up after receipt of the various gene therapies is relatively short, most patients have experienced resolution of VOEs and improved hemoglobin levels (>11 g/dL). However, mild, sub-clinical hemolysis may persist after otherwise successful gene therapy^1,2

A serious concern regarding cellular therapies for SCD is the potential for myeloid malignancies to develop after treatment. At least two patients in the initial lovo-cel trial developed acute myeloid leukemia.^4,5 SCD itself may be a risk factor for developing myeloid malignancies,^18–20 a risk that might increase after the reinfusion of autologous gene-modified HSCs,²¹ in addition to chemotherapy exposure. However, the true incidence of this outcome is unknown and will become apparent only with longer follow-up. Some of the risks associated with conditioning, such as infertility and organ dysfunction, are present with the use of all genetic therapies that require myeloablative chemotherapy.

In addition to the above concerns, there are certain procedural burdens to be aware of, starting with the 6–12 months long potential timeline from initial discussion to the actual administration of gene-modified cells. This journey includes:

• Screening procedures.

• Discontinuation of disease modifying therapies such as hydroxyurea.

• Repeated blood transfusions including erythrocytopheresis to reduce risk of vaso-occlusive events during apheresis and to optimize CD34 collection.

• Central venous catheter insertion when peripheral access is inadequate for exchange transfusions and/or apheresis and for transplant admission.

• Apheresis to collect CD34+ cells. Some patients may require multiple collections. Furthermore, despite multiple attempts, it may prove impossible to collect sufficient CD34+ HSCs from some patients to manufacture a product.

• Fertility preservation.

• Variable wait times for manufacturing of gene therapy product which can last 3–6 months.

• Admission for conditioning, stem cell infusion, and recovery.

• Several months of outpatient follow-up before normal activities can be resumed.

Figure 1 provides a schematic summary of the entire journey from initial decision to pursue gene therapy to long term follow up. Table 1 highlights the key differences in the current approach to curative therapy for SCD.

Figure 1. Clinical course of an individual receiving autologous gene therapy for sickle cell disease.

First, candidates interested in pursuing autologous gene therapy are screened for eligibility by reviewing their medical history, laboratory investigations, and imaging to assess adequacy of their organ function, active infections, and overall fitness to receive myeloablative chemotherapy. Individuals who are deemed eligible and fit to receive autologous gene therapy then discontinue hydroxyurea treatment and begin simple or exchange transfusions, which are generally continued for at least 2 to 3 months before mobilization of hematopoietic stems cells, and apheresis collection are attempted. Each apheresis cycle comprises 3 to 4 consecutive days of daily collections. If adequate CD34+ cells are not collected in one cycle, additional collection cycle may need to be performed. Successive cycles of apheresis are generally separated by at least 4 to 6 weeks to enable the patient to recover from the previous procedure completely and to continue transfusions in the interim. The harvested apheresis packs are shipped to the manufacturing site where they undergo CD34+ cell isolation followed by transduction with a lentivirus or electroporation with Cas9 and guide RNA. The final manufactured product is cryopreserved while release testing and quality control studies are performed. After the completion of the quality testing, if the product meets the specified minimum criteria, it is shipped to the clinical site that will be delivering the product to the patient. The patient is admitted to the inpatient unit and myeloablative conditioning is administered. After a washout period (usually 24–48 hours after busulfan treatment), a genetically modified cellular product is infused into the patient via a central venous catheter. Recipients usually remain inpatient until engraftment and are then followed regularly to evaluate them for hematopoietic recovery, any residual SCD-related complications, and late effects of chemotherapy exposure. Recipients of autologous genetically modified products are then followed prospectively for several years to assess for durability of the treatment and development of any adverse events. (Figure reprinted from Publication title: Blood, Vol /edition number 144, Author(s): Akshay Sharma, Title of article / title of chapter: How I treat sickle cell disease with gene therapy, Pages No. 269 3–2705, Copyright (Year) 2024, with permission from Elsevier.)

Table 1.

Pros and cons of the potentially curative treatment options available for patients with SCD.

Allo-HCT	Mechanism	Pros	Cons
General	Multiple (listed below)	Potential for cure Large cumulative experience Reduces progressive vasculopathy and end-organ damage Increasing donor availability Lower cost than gene therapy	Chemotherapy +/− radiation related short term and late effects including compromised fertility
Matched sibling donor	Myeloablative (MAC) Non-myeloablative (NMA)	Myeloablative (mainly used in children) 5-year EFS: >90%; OS: >95% Extended followup: >15 years Non-myeloablative (Better tolerated in adults) 5-year EFS: >85%; OS: >90% Extended followup: >15 years Less chronic GvHD	Limited donor availability (8–14% of patients with SCD) Graft rejection in 5–30%; TRM in 5–10% (MAC) Acute GvHD (~20%) Chronic GvHD (~10%) Late effects: pulmonary toxicity, infertility, endocrine and metabolic issues Mortality related to transplant (<5%) With NMA >50% with mixed chimerism, increasing risk of graft failure, and myeloid malignancy risk
Haploidentical BMT with post-transplant cyclophosphamide	Non-myeloablative	Reduced acute and chronic GvHD (<10%) Less complex cell manipulation EFS: >85%; OS: >95%	Potential for acute and late toxicities from high doses of cyclophosphamide Increased risk of graft rejection (25% in children) Increased risk of viral reactivation
T-cell receptor alpha/beta+ and CD19+ depletion	Myeloablative (MSD and haploidentical donors)	EFS: >85%; OS: >95% Lower risk of grade III/IV acute GvHD and severe chronic GvHD	Delayed immune reconstitution compared to T-replete grafts Requires cell processing expertise More costly Available at fewer centers
Gene Therapy	Mechanism	Pros	Cons
General	Myeloablative	Limited to patients with adequate organ function No donor needed (autologous) Avoids immunologic complications (GvHD or graft rejection)	Requires multiple apheresis cycles for cell collection Requires a GMP facility for manufacturing Acute and chronic toxicities associated with high-dose busulfan Poor phenotypic (rheologic and hemolytic) correction Risk of genotoxic side effects Unknown impact on progressive vasculopathy and end-organ damage in adults Cost of the product itself is ~5× that of an allo-HCT
CRISPR-Cas based5	HbF induction by disruption of BCL11A erythroid enhancer HbF induction by disruption of BCL11A binding at the gammaglobin promoter Direct correction of the sickle mutation	No integration of viral vectors	Requires introduction of double stranded breaks in the genome Potential for off-target editing
Lentiviral Vector Gene Addition6	Lentiviral vector encoding either a human γ-globin gene or a normal or modified β-globin gene designed for anti-sickling activity Lentiviral vector encoding a short hairpin RNA molecule for post-transcriptional silencing of BCL11A	Can transduce non-dividing HSCs with high efficiency	Semi-random integration leading to potential off-target effects or insertional mutagenesis

Abbreviations: allo-HCT: allogeneic hematopoietic stem cell transplant; GvHD: graft-versus-host disease; MAC: myeloablative conditioning; NMA: non-myeloablative; TRM: treatment-related mortality; GMP: Good Manufacturing Practice

In practice, the choice between allo-HCT and gene therapy is dynamic rather than binary. Factors such as age, organ function, prior treatment history, reproductive goals, donor availability, and insurance coverage all influence the optimal approach. As longer-term data emerge for gene therapy, periodic reassessment of curative options over the lifespan will be important, particularly for adolescents transitioning to adult care.

How do you prepare a patient to undergo stem cell collection for gene therapy?

Before hematopoietic stem progenitor cell (HSPC) mobilization and collection, disease-modifying agents, particularly hydroxyurea, should be stopped to reduce myelosuppression and promote the yield of CD34⁺ cells.²² Also, patients should receive RBC exchange transfusions for a minimum of 2–3 months to optimize the collection yield. Automated red cell exchange (RCE) transfusion is preferred, although a single automated exchange transfusion followed by monthly simple or exchange transfusions is also acceptable, provided that the hemoglobin S fraction is maintained at below 30% (often ≤ 20% preferred).^22–24 Because some patients may develop allo-antibodies with exposure to transfused units, acquiring adequate blood units to perform RCE may be challenging. A formal evaluation by a transfusion medicine specialist, and frequent communication regarding RCE frequency and hemoglobin targets is critical.

How to achieve best outcomes during apheresis?

Target cell dose:

Obtaining sufficient HSPCs from patients with SCD is critical for successful gene therapy. Because the cell dose target is relatively high, most centers perform large-volume leukapheresis (greater than 4 total blood volumes [TBVs]), which may take 6–8 hours on 2–4 consecutive days. Despite this, most patients require more than 2 cycles of collection, each lasting several days. However, large-volume leukapheresis and leukapheresis exceeding 4–5 hours can increase the risk of VOE, and patients should be monitored closely. In addition to transfusions to reduce HbS%, strict fluid balance and electrolyte replacement should be maintained during leukapheresis to reduce the risk of complications.²⁵

Timing of plerixafor administration and mobilization kinetics:

Granulocyte colony-stimulating factor (G-CSF) is contraindicated in individuals with SCD because of the potential for life-threatening leukocytosis and neutrophil activation.^26,27 Extreme caution should be exercised when using outside a clinical trial setting. Plerixafor is safe and effective for HSC mobilization as a single agent. It should be administered 2–4 hours before leukapheresis, as the peak peripheral CD34⁺ counts are expected between 3 and 6 hours post plerixafor in individuals with SCD.²⁴ This administration may be repeated daily for up to 2–4 days per collection cycle, as tolerated by the patient and per institutional standards. If further cycles are required to collect additional CD34⁺ cells, we recommend spacing the subsequent collection attempts by at least 4 weeks.

Collection procedure optimization:

Leukapheresis in SCD can be particularly challenging, as patients have altered RBC morphology and hypercoagulability, resulting in high levels of RBC–neutrophil/monocyte and platelet–neutrophil/monocyte aggregates that affect the cell separation during leukapheresis.²³ This leads to difficulty in establishing and sustaining an interface between different layers during leukapheresis. Targeting a deeper collection depth can help capture more CD34⁺ cells,^23,28 although doing so can increase the RBC contamination. Increasing the packing factor can also help improve the RBC sedimentation. The optimal anticoagulant for use during leukapheresis is not yet defined; commonly, anticoagulant citrate dextrose solution A (ACD-A) alone is used, although some centers use a combination of ACD-A and heparin, enabling higher inlet flow rates and less ACD-A exposure.²⁹ Some centers also give patients daily aspirin before collection to reduce micro-clotting, but this practice has not been systematically studied.²⁴ It is strongly recommended that apheresis for individuals with SCD should be performed at centers with established experience in SCD-specific mobilization, standardized order sets, and protocols for rapid recognition and management of VOEs during collection.

What happens when adequate HSC collection is not achieved?

It can be difficult to collect sufficient HSCs because of multiple factors, including older age, systemic and bone marrow inflammation, disease severity, and collection limitations.^22,30 Although plerixafor continues to be widely used as a single agent for stem cell mobilization for SCD, motixafortide, a CXCR4-inhibitory peptide with high binding affinity for CXCR4, is currently being studied in patients with SCD as its use may result in higher and more sustained levels of mobilized CD34⁺ cells.^31,32 Loss of HSCs during manufacturing is a barrier to gene therapy that may require technological advances to overcome.

What are the manufacturing factors that affect the safety and efficacy of ex vivo gene therapy in SCD?

Standard critical quality attributes (CQAs) for gene therapy products include their purity, identity, potency, safety (as determined by sterility assays, endotoxin assessment, and mycoplasma testing), and stability.^33,34 As part of the safety assessments of LVV-based gene insertions, replication-competent lentivirus (RCL) testing is required on the vector producer cells, on the end-of-production cells, on the vector stock in the cell culture supernatant, and in the LVV-modified patient cells. Current recommendations include testing samples from multiple stages of the manufacturing processes.³⁵ Integration site analysis (ISA) monitoring for clonal expansion is recommended periodically but no sooner than 6 months post infusion.³⁶

For gene editing strategies, although not for the release of individual products, complete off-target editing analysis (by homology-based unbiased methods to nominate off-target sites, followed by targeted next-generation sequencing for confirmation)^37,38 and karyotype analysis are required by regulatory authorities to confirm the safety of the product before approval.

The total HSPC dose, the vector copy number (VCN), or average number of gene insertions in the HSPCs, and the percentage of HSPCs edited/corrected in the drug product are important manufacturing parameters that affect efficacy. A higher cell dose, VCN, or percentage of LVV-modified/gene-edited cells lead to better outcomes. Of note, a higher VCN theoretically increases the risk of insertional mutagenesis as well. The optimal VCN or gene editing frequency in vivo for maximal phenotype reversal has not yet been determined and probably depends on the specific vector biology. A standardized potency assay that includes a functional assessment of the effect of the drug product on sickle cells has been required for the release of commercial products.

What fertility preservation methods are used before treatment?

Fertility preservation is indicated for those patients who have not completed their families and are undergoing allo-HCT or gene therapy. Although fertility-preserving conditioning regimens for allo-HCT based on antibody–drug conjugates are under investigation,³⁹ gene therapy is currently universally performed using the gonadotoxic alkylating agent busulfan.^1,2 Busulfan exposure is a risk factor for infertility and gonadal failure.^17,40–42 Individuals with SCD inherently may have diminished ovarian reserve, semen abnormalities, and infertility.^43,44 Pretreatment counseling regarding baseline gonadal function, ovarian reserve, semen analyses, partner testing for hemoglobinopathy trait, infertility, and fertility preservation options is imperative.^45,46 Clear genetic counseling is critical to ensure that patients are fully informed that gene therapy does not resolve the heritability of sickle cell or thalassemia trait, which is passed by 100% of the gametes of affected individuals.

Two standard fertility-preservation interventions for people with ovaries are ovarian tissue cryopreservation (OTC) and ovarian stimulation with oocyte or embryo cryopreservation. Before puberty, OTC under anesthesia is the only option.⁴⁷ Reimplantation of cryopreserved ovarian tissue presents opportunities for restoring gonadal function and the possibility of future oocyte development and pregnancy.⁴⁸ To achieve pregnancy with cryopreserved ovarian tissue, in vitro fertilization (IVF) is usually required.

Ovarian stimulation takes 2–3 weeks to complete and is followed by transvaginal oocyte harvesting under brief, light anesthesia. Ovarian stimulation planning is imperative, as there are many potential SCD-related issues, including the development of VOEs, thromboses, complications of steroid exposure, and infections.^49–51 Ovarian hyperstimulation syndrome is uncommon, but vigilant post-procedure monitoring is warranted.^49,51 Gametes or embryos that are cryopreserved can later be used to pursue pregnancy via IVF. Whether or not to discontinue hydroxyurea before OTC or ovarian stimulation is not yet established.⁴⁶ Because fertility preservation is a high-stakes consideration and the impact of hydroxyurea on oocyte development or future fertilization is unclear, a treatment pause should be considered.⁴⁹

For sperm preservation, options include ejaculated sperm or, for prepubescent patients, testicular tissue cryopreservation.⁴³ Because oligospermia may occur in patients with SCD, particularly in those receiving hydroxyurea, multiple ejaculated samples may be needed. In addition, a wash-out period after stopping hydroxyurea is usually needed to allow sperm recovery.⁵² Intrauterine insemination or IVF are used for conception with cryopreserved sperm.

Long-term chronic transfusion therapy may protect the ovaries and testes, thereby guard the ovarian reserve and improve semen parameters and testosterone levels.⁵³ Given the complexity and emotional weight of fertility decisions, referrals to reproductive endocrinology and psychology should be initiated early in the gene therapy evaluation process, ideally before mobilization planning, to allow patients and families adequate time to consider options.

What factors affect busulfan conditioning?

After the insertion of a central venous catheter and pre-conditioning RCE transfusion aimed at achieving a hemoglobin S fraction of <30%, patients are admitted to the hospital for myeloablative chemotherapy, for infusion of the genetically modified cells, and through recovery. Patients receive single agent busulfan for 4 days, with the dosing being adjusted according to pharmacokinetic (PK) monitoring to ensure safe and effective exposure. In allo-HCT for hemoglobinopathies, the optimal myeloablative cumulative target exposure for Bu AUC_{0–4 days} is 90 mg*h/L (range: 80–100 mg*h/L) over 4 days. In gene therapy clinical trials, participants received AUC_{0–4 days} 59–100 mg*h/L over 4 days. Underexposure is associated with graft failure in allo-HCT or with less engraftment of gene-marked cells in the case of gene therapy, which may result in suboptimal correction of the underlying disease. Overexposure is associated with increased organ toxicity. The main source of PK variation is the different models used.^54–57 Whereas some centers employ specialized PK software with validated models, others use non–model-based trapezoidal methodology for exposure calculation, which may result in substantially biased AUC exposures.^54,55 Using a validated population PK model to estimate AUC enables limited sampling strategies while providing a more accurate estimate of true drug exposure. This is because validated population PK models incorporate the exact timing of infusion, account for sampling and analytical variability, and use individual clearance to estimate exposure. In addition to busulfan, another alkylating agent, melphalan, has been used as a conditioning agent for gene therapy.⁵⁸ However, melphalan lacks a validated population PK model, and its optimal exposure remains unknown.

What are the common complications of busulfan conditioning?

Management of patients with SCD undergoing gene therapy, including pain management, transfusion support, and other SCD related factors are distinct from patients undergoing autologous HCT for other indications. Typically, patients are hospitalized for 4–6 weeks for chemotherapy administration and recovery after the infusion of the genetically modified cells before transitioning to the outpatient clinic.

• Infection: Patients should receive prophylactic antimicrobial coverage and practice infection precautions until they experience full immunological recovery. The exact impact of busulfan conditioning on infection risk (other than during the short period of neutropenia and related infections), including whether there is a need for re-vaccination, remains to be determined.⁵⁹

• Mucositis ± pain management: The trigger of tissue injury in mucositis can amplify pain and require extended analgesia with tapering over several months, especially in those patients with chronic pain. Developing an individualized pre-transplant analgesia plan with input from the patient and pain specialists is recommended.

• Sinusoidal obstruction syndrome (SOS; formerly known as veno-occlusive disease [VOD]) SOS/VOD of the liver is a complication of myeloablative chemotherapy, especially associated with busulfan conditioning, which is not uncommon in individuals with SCD. It is more common in those with liver fibrosis, which may be seen subsequent to iron overload from frequent transfusions.

• Nutritional support: As after an allo-HCT, a nutritional plan is needed to support patients through the mucositis period. This may include supplements, enteral feeding plan or use of total parenteral nutrition.

• Transfusion support: Packed red blood cell and platelet transfusions are administered as needed to maintain hemoglobin between 9 and 11 g/dL post infusion to avoid hyperviscosity and acute anemia. The platelet count is maintained at ≥50,000/μL to prevent intracranial hemorrhage and other serious bleeding. Alloimmunization can make maintaining transfusion support challenging, and transfusion medicine consultation is recommended before initiating conditioning.

What are psychosocial and biobehavioral considerations before and after gene therapy administration?

• Complexity of SCD Pain:

  SCD pain has multifactorial etiologies and may persist even after gene therapy as it may stem from accumulated organ damage (such as avascular necrosis) or be related to central sensitization due to years of recurrent pain episodes.^60,61 Patients may have developed opioid tolerance; therefore, long-term weaning plans and other treatment options may be needed if chronic pain persists. Multidisciplinary management including behavioral specialists and pain specialists should be instituted before transplant and continue post treatment with a well-defined follow-up plan.

• Post-gene therapy - “The New Normal”:

  Individuals with SCD experience repeated hospitalizations, social disruptions, and anxiety about life expectancy from birth. After undergoing a transformative therapy, it can be difficult for them to grasp how they should feel or what pain is ―normal‖ relative to their pre-treatment perspective.⁶² Furthermore, people with SCD may face new psychosocial or familial pressures after treatment, and may struggle to redefine their identity after years of identifying as a person with a chronic illness and not understand how to adjust post treatment. They may require psychological rehabilitation and counseling to help define their new identity. Overall, patient education is essential to help individuals transition from SCD-focused care to wellness-centered living.

What is the plan for long-term monitoring?

The timing and scope of surveillance for complications following autologous gene therapy have not been well defined. We recommend annual surveillance beginning 1–2 years after treatment, informed by long-term follow-up practices after allo-HCT and up to 15 years after infusion as per FDA recommendations.^36,63,64 Long-term follow-up should be conceptualized across three domains: (1) late effects of conditioning (e.g., gonadal failure, secondary malignancy), (2) risks related to gene modification (insertional mutagenesis or off-target editing), and (3) evolution of SCD-related organ damage and health-related quality of life.

Busulfan exposure:

A myeloablative dose of busulfan places patients at risk for gonadal toxicity (discussed above) and secondary cancer. The mechanism of secondary cancer development in individuals with SCD is not well understood. In an observational cohort of 1096 recipients of allo-HCT for 6631 person-years, the 10-year incidence of any secondary cancer was 2.4%.⁶⁵ In the subset aged <12 years, five of eight cancers were solid secondary cancers and occurred after high-dose busulfan treatment.⁶⁵ In another registry study comprising of 300 individuals with SCD who underwent allo-HCT, the standardized incidence ratio for development of secondary neoplasms was 11, which is significantly higher than the expected incidence in the general population.⁶⁶ This, together with the occurrence of leukemia after allo-HCT with high-dose busulfan, supports lifelong cancer surveillance.⁶⁷ Such surveillance may include an annual medical history targeted towards cancer and further investigations initiated when signs are suggestive for oncogenesis.³⁶

Induced mutagenesis:

A major concern with LVV therapy is that the genetic product may deleteriously integrate into the genome. Genome-editing technologies are associated with a risk of off-target changes in the genome, resulting in aberrant gene expression. Monitoring should consider these product-specific characteristics. This may be achieved via an annual medical history and examination targeted towards screening for cancer and hematologic disorders. Integration site analysis or indel analysis may be used to monitor for clonal expansion associated with a specific integration site after LVV transduction or with a specific indel pattern after CRISPR-Cas9 editing, respectively.³⁶ Validated methods need to be developed for these assessments to be used in the clinical setting.

Organ function assessment:

Routine health evaluations may be focused on organ dysfunctions, which may predate the receipt of gene therapy or develop during its course. Age-appropriate SCD-related evaluations and follow-up should also continue until long-term efficacy data for gene therapy products are available.

HIV testing post lentiviral therapy:

Caution is advised for patients undergoing human immunodeficiency virus (HIV) testing post treatment with lovo-cel, as some standard tests may yield false-positive results, leading to unnecessary patient anxiety.^68,69 The use of alternative HIV tests such as HIV-1 proviral DNA PCR (qualitative nucleic acid based test) is recommended.

Participation in national and international registries with harmonized data elements will be essential to define the true long-term benefit–risk profile of these therapies and to inform future updates of these recommendations.

How are these therapies being paid for?

Several aspects must be considered when evaluating financial factors associated with treatment. The upfront costs of undergoing gene therapy, along with hidden expenses (such as the cost of travel to the gene therapy center or of lodging if away from home) and follow-up costs associated with complications and delayed effects, are significant. The cost of gene therapy products is considerable ($2.2 million for exa-cel to $3.1 million for lovo-cel for a one-time infusion), but they are believed to be reflective of the lifetime cost of caring for someone with SCD. For most patients who live in low- or middle-income countries, their options are severely restricted by these prohibitive costs. Even in high-income countries, the lump-sum payment, the fragmentation of payers and coverage, and the uncertainty regarding treatment durability remain a challenge for payers. In the United States, the Center for Medicare and Medicaid Innovation (CMMI) and the Centers for Medicare & Medicaid Services (CMS) have launched the Cell and Gene Therapy Access Model specifically targeting SCD to address these very challenges. This model would organize price negotiations on behalf of participating state Medicaid agencies and manufacturers and would incorporate unified data tracking using registries, outcomes-based payments, patient-centered benefits (such as payment for fertility preservation), and support for patient education and case management.^70–72 In parallel with innovative payment models, investment in comprehensive SCD care, HCT and gene therapy infrastructure, and patient navigation will be required to ensure that advances in genetic therapies translate into real-world gains in survival and quality of life for populations that have been historically underserved.

Conclusions

Gene therapies have changed the treatment paradigm for people with SCD. Despite being the first molecular disease to be identified, our understanding of the complex pathophysiology of SCD and its definitive treatments has lagged compared to other genetic diseases that were identified much later. Although the development of these gene therapies represents a welcome change in the pace of development of therapeutic interventions for this neglected disease, it has brought many novel challenges. Issues related to access and equity must be addressed so that the injustice and medical neglect faced by generations of people with SCD can be addressed. Patient-centered and culturally appropriate educational materials are essential to disseminate accurate information about these novel therapies and address common misconceptions. Long-term follow-up in a comprehensive clinic—staffed by an SCD specialist, a HCT specialist, and a mental health professional experienced in the challenges associated with these transformative therapies—is critical for monitoring late-onset complications and residual organ dysfunction that may have accumulated prior to treatment. In addition to routine medical management, regular assessments of quality of life, pain, mental health, and social reintegration can inform personalized care plans. A holistic, multidisciplinary, patient-centered approach is fundamental to achieving durable outcomes in this new era of SCD care.

Table 2.

A summary of the key recommendations.

1.	Gene therapy for SCD requires multidisciplinary evaluation, integrating care by hematology, transplant, transfusion medicine, reproductive health, mental health, and social service providers to ensure patients can safely complete the full treatment pathway.
2.	Patient selection should extend beyond genotype and disease severity, incorporating organ function, comorbidities, reproductive goals, psychosocial context, and capacity for long-term follow-up.
3.	Plerixafor-only mobilization is recommended, with granulocyte colony-stimulating factor contraindicated due to safety concerns in SCD.
4.	Automated red cell exchange before mobilization and conditioning is essential to reduce hemoglobin S levels and minimize peri-procedural complications.
5.	Myeloablative busulfan conditioning remains standard, requiring pharmacokinetic-guided dosing and proactive management of toxicity, fertility risk, and infection.
6.	Fertility preservation counseling and intervention should occur early, as current gene therapy approaches are gonadotoxic and do not alter genetic transmission risk.
7.	Long-term, lifelong surveillance is mandatory, addressing late effects of conditioning, risks related to gene modification, and the evolution of SCD-related organ damage.
8.	Participation in national and international registries is strongly encouraged to define durability, late toxicities, and real-world effectiveness of gene therapies.