Gene therapy for sickle cell disease reduced symptoms in 3 patients after process improvements; long-term expression remains a challenge
Fewer symptoms and less transfusion burden after process improvements
In a phase 1/2 lentiviral gene therapy trial involving four adults with severe sickle cell disease, the three patients treated after improvements to cell collection and gene delivery had fewer vaso-occlusive crises and a lower transfusion burden. However, vaso-occlusive crises did not disappear, and sustained expression of the therapeutic hemoglobin varied among patients. The study shows that, in addition to gene delivery, long-term engraftment and sustained expression remain challenges that shape treatment outcomes.
Adding a therapeutic gene to patients’ own cells
Sickle cell disease is caused by HBB gene mutations that produce abnormal hemoglobin HbS. Under low-oxygen conditions, HbS polymerizes and makes red blood cells rigid and misshapen, leading to vaso-occlusion and red blood cell destruction, or hemolysis. Autologous gene therapy uses a patient’s own hematopoietic stem and progenitor cells in an approach intended to avoid the constraints of donor availability and the risk of graft-versus-host disease.
This single-center study at UCLA in the United States was a nonrandomized, open-label trial that reported outcomes through 24 months after transplantation in four patients aged 25–29 years. Before treatment, patients had 11–12 vaso-occlusive crises per year and were receiving regular red blood cell transfusions. The primary endpoints were safety and the feasibility of manufacturing the gene-modified cell product; the study also assessed gene marking, therapeutic hemoglobin expression, and clinical outcomes.
The research team used a lentiviral vector to add a βAS3-globin gene designed to inhibit sickling of red blood cells. This differs from gene editing that directly corrects the existing HBB mutation. After busulfan conditioning, patients received infusions of their own gene-modified cells.
Vector persistence and gene activity were not the same
The first patient was treated with cells collected from bone marrow and the initial vector. After transplantation, gene marking was low and HbAS3 protein was undetectable, and clinical benefit was limited.
Subsequent process changes included introducing transfusions and red blood cell exchange before collection and collecting cells from peripheral blood mobilized with plerixafor. The vector was redesigned to have a smaller structure, and a gene-transfer enhancer was also used. Because multiple elements were changed together, this trial cannot isolate the contribution of each change.
With the improved process, peak vector copy numbers (VCNs) in granulocytes among the three treated patients were approximately 0.5–2.0. VCN indicates the number of vector copies per cell; this does not mean that values remained within this range throughout follow-up. One patient had relatively stable gene marking and HbAS3 expression through 24 months. In another, therapeutic gene expression fell substantially despite persistent vector marking; in the remaining patient, both vector marking and expression declined. The researchers suggested transcriptional suppression as a possible explanation for declining expression despite persistent vector marking, and insufficient long-term engraftment or loss of gene-modified cells for the decline in both marking and expression, but did not establish the causes.
Scope of clinical improvement and remaining limitations
Among the three patients treated with the improved process, annual vaso-occlusive crises fell from 12 to 3, from 11 to 1.5, and from 12 to 2, respectively. The abstract reported that two of these patients became transfusion-independent, but did not specify how long that status lasted. Differences in clinical measures before and after treatment across all four patients were not statistically significant.
All four patients recovered hematopoietic function without rescue transplantation. No vector-related serious adverse events or insertion-related tumor formation were observed. However, adverse events consistent with busulfan administration occurred, and one patient developed grade 3 mucositis. Two years of observation in a small number of patients cannot establish long-term safety.
The study ended early, and further development of this vector will not continue. The researchers cited the availability of other approved gene therapies that had shown greater efficacy in clinical trials as the reason for ending the study. This was not the result of a direct comparison of treatments in this trial. Within the limitations of its small, nonrandomized design, the study offers a development lesson: optimization needs to link cell manufacturing with engraftment and sustained expression.
Sources: Original article in Blood Advances, PubMed abstract.
Sickle cell disease (SCD) is a monogenic disorder where autologous gene therapy may offer a safer curative alternative to allogeneic transplantation. We report outcomes from a Phase I/II study using the Lenti/G-βAS3-FB lentiviral vector, encoding an anti-sickling β-globin. This trial was registered at ClinicalTrials.gov (NCT02247843). This single-site study treated four adults with severe SCD. The first patient was treated using the original Lenti/βAS3-FB and initial protocol, which resulted in suboptimal clinical response. Subsequent protocol refinements included improved hematopoietic stem and progenitor cell (HSPC) collection using plerixafor‑mobilized peripheral blood apheresis with pre‑collection erythrocytapheresis, and use of an optimized lentiviral vector with a transduction enhancer. All patients received myeloablative busulfan conditioning followed by infusion of gene‑modified autologous HSPCs. Primary endpoints were safety and feasibility; secondary endpoints included gene marking, therapeutic hemoglobin expression, and clinical outcomes. All patients achieved hematopoietic recovery without rescue transplantation. The first patient demonstrated low gene marking (peak vector copy number [VCN] 0.035) and undetectable HbAS3 expression, with minimal clinical benefit. In contrast, the three patients treated with the optimized protocol achieved higher and sustained gene marking (peak granulocyte VCNs ~0.5‑2.0) and persistent HbAS3 expression. These patients experienced some reductions in vaso‑occlusive crises and transfusion requirements, with two becoming transfusion‑independent. No insertional oncogenesis was observed. This trial highlights the necessity of optimized vector design and transduction protocols to achieve durable gene expression. While this specific vector will not be pursued further, the study provides crucial insights into gene therapy protocol development.
This study shows that introducing a therapeutic gene into cells and keeping that gene sufficiently active over the long term are separate challenges. In one patient, expression fell despite persistent vector marking, making it difficult to judge treatment durability from VCN alone. For researchers, this means tracking long-term engraftment, RNA and protein expression, and clinical course alongside cell collection and manufacturing conditions. Because several process changes were made at once, the effect of any single improvement cannot be established. Reduced symptoms and transfusion burden in some patients are signals relevant to further development, but conditioning toxicity and incomplete effects remain; the findings cannot be interpreted as evidence of a safe cure.