Background
Two ongoing phase 3, open-label, single-group studies (CLIMB THAL-141, CLIMB SCD-151) in children 5–11 years of age. Exagamglogene autotemcel (exa-cel) is a one-time autologous cell therapy in which CD34+ haematopoietic stem cells undergo ex vivo CRISPR-Cas9 editing of the erythroid-specific enhancer of BCL11A, reactivating fetal haemoglobin (HbF). N=26 children: 15 with transfusion-dependent β-thalassemia (TDT) and 11 with sickle cell disease (SCD) with recurrent vaso-occlusive crises. Prior phase 3 data established efficacy in patients 12–35 years; this study extends exa-cel below the currently approved ≥12-year age.
Results
Interventions and follow up: Design: single-arm, no comparator (each child serves as own historical control).
Conditioning: pharmacokinetically dose-adjusted myeloablative busulfan.
Exa-cel: single intravenous infusion of autologous BCL11A-edited CD34+ cells.
Primary endpoint (TDT): transfusion independence for ≥12 consecutive months.
Primary endpoint (SCD): freedom from severe vaso-occlusive crises for ≥12 consecutive months.
mFollow up: 16.0 mo (TDT, range 2.2–32.1); 16.9 mo (SCD, range 7.6–33.1)
Results: TDT – transfusion independence: 8/8 children followed ≥16 mo were transfusion-independent (7 of 15 not yet evaluable).
SCD – severe-VOC freedom: 8/8 children followed ≥16 mo were free of severe vaso-occlusive crises (3 of 11 not yet evaluable).
Maturity: interim readout — formal ≥12-month primary endpoints (TI12 / VF12) await follow-up of the remaining children.
Conditioning: pharmacokinetically dose-adjusted myeloablative busulfan.
Exa-cel: single intravenous infusion of autologous BCL11A-edited CD34+ cells.
Primary endpoint (TDT): transfusion independence for ≥12 consecutive months.
Primary endpoint (SCD): freedom from severe vaso-occlusive crises for ≥12 consecutive months.
mFollow up: 16.0 mo (TDT, range 2.2–32.1); 16.9 mo (SCD, range 7.6–33.1)
Results: TDT – transfusion independence: 8/8 children followed ≥16 mo were transfusion-independent (7 of 15 not yet evaluable).
SCD – severe-VOC freedom: 8/8 children followed ≥16 mo were free of severe vaso-occlusive crises (3 of 11 not yet evaluable).
Maturity: interim readout — formal ≥12-month primary endpoints (TI12 / VF12) await follow-up of the remaining children.
Adverse events
Grade 3 or 4 (any): 100% — all 26 children had ≥1 grade 3/4 adverse event.
Conditioning-related serious: severe hepatic veno-occlusive disease (VOD/SOS) in 2 TDT children, both attributed to busulfan.
Deaths: 1 (TDT child with busulfan-related severe veno-occlusive liver disease).
Context: toxicity reflects myeloablative busulfan conditioning (cytopenias, infection risk, VOD) rather than the edited cells, consistent with autologous HSCT.
Conditioning-related serious: severe hepatic veno-occlusive disease (VOD/SOS) in 2 TDT children, both attributed to busulfan.
Deaths: 1 (TDT child with busulfan-related severe veno-occlusive liver disease).
Context: toxicity reflects myeloablative busulfan conditioning (cytopenias, infection risk, VOD) rather than the edited cells, consistent with autologous HSCT.
Conclusions
Exa-cel produced transfusion independence (TDT) and freedom from severe vaso-occlusive crises (SCD) in all evaluable children 5–11 years followed ≥16 months, extending a one-time curative CRISPR therapy below the current ≥12-year label. However, every child had a grade 3/4 event and one died of busulfan-related veno-occlusive liver disease, underscoring that myeloablative conditioning — not the gene editing — remains the principal hazard. Longer follow-up and the not-yet-evaluable children will define durability and the complete safety profile.
Key Limitations
Small, single-arm cohorts (15 TDT, 11 SCD) with no concurrent comparator; benefit is judged against each child’s own history rather than allogeneic HSCT or best supportive care. Interim/immature data — many children not yet evaluable for the ≥12-month primary endpoints. Open-label. Median follow-up (~16 mo) is short for a one-time therapy where durability, late effects, fertility (busulfan gonadotoxicity) and secondary-malignancy risk require years of surveillance. One treatment-related death and 2 severe VOD events highlight conditioning toxicity; less-toxic / non-genotoxic conditioning remains an unmet need. Industry-funded (Vertex Pharmaceuticals / CRISPR Therapeutics).
Clinical Context
Exa-cel (Casgevy) is the first approved CRISPR-Cas9 gene-edited therapy; FDA-approved for patients ≥12 years — sickle cell disease with recurrent VOCs (Dec 2023) and transfusion-dependent β-thalassemia (Jan 2024) — with EMA conditional marketing authorization (2024). These CLIMB THAL-141 / SCD-151 data extend efficacy and safety to children 5–11 years and support label expansion to younger patients. For TDT, exa-cel offers a one-time alternative to lifelong transfusion plus iron chelation or matched-donor allo-HSCT; for SCD, an alternative to chronic transfusion, hydroxyurea and other disease-modifying agents or allo-HSCT. Curative intent is weighed against myeloablative busulfan toxicity, access and cost; matched-sibling allo-HSCT remains an option where a donor exists.