Background
Phase III single-arm, open-label study (CLIMB-SCD-121), N=44, age 12-35 with severe sickle cell disease (HbSS or HbSβ0) and ≥2 severe vaso-occlusive crises (VOC)/yr for the 2 years before screening. Exa-cel (exagamglogene autotemcel; Casgevy) uses CRISPR-Cas9 editing of autologous CD34+ HSPCs at the BCL11A erythroid enhancer to reactivate fetal hemoglobin (HbF). FDA approved December 8, 2023; first FDA-approved CRISPR-based therapy.
Interventions and follow up
Treatment: single IV infusion of exa-cel (≥3×10⁶ CD34+ cells/kg) after myeloablative busulfan conditioning
Primary endpoint: freedom from severe VOC for ≥12 consecutive months
mFollow up: median 19.3 months (range 0.8-48.1)
Primary endpoint: freedom from severe VOC for ≥12 consecutive months
mFollow up: median 19.3 months (range 0.8-48.1)
Results
Freedom from severe VOC ≥12 months: 29/30 evaluable (97%; 95%CI 83-100), P<.001 vs null of 50%
Freedom from VOC hospitalization ≥12 months: 30/30 (100%; 95%CI 88-100), P<.001
HbF: median increased from <1% to >40% post-infusion
Engraftment: achieved in all 44 patients
Freedom from VOC hospitalization ≥12 months: 30/30 (100%; 95%CI 88-100), P<.001
HbF: median increased from <1% to >40% post-infusion
Engraftment: achieved in all 44 patients
Adverse events
Neutropenia: 100%
Thrombocytopenia: 100%
Other: mucositis, febrile neutropenia (expected from busulfan conditioning/autologous HSCT)
Sinusoidal obstruction syndrome: 1 patient (treated successfully)
Malignancy/off-target editing: none detected
Treatment-related deaths: none
Thrombocytopenia: 100%
Other: mucositis, febrile neutropenia (expected from busulfan conditioning/autologous HSCT)
Sinusoidal obstruction syndrome: 1 patient (treated successfully)
Malignancy/off-target editing: none detected
Treatment-related deaths: none
Conclusions
A single exa-cel infusion eliminated severe VOC in 97% of evaluable patients with severe sickle cell disease, with 100% free from VOC hospitalization, representing a functional-cure approach via durable CRISPR-Cas9-mediated HbF reactivation.
Key Limitations
Single-arm, no comparator; small N with only 30 evaluable for the primary endpoint; limited follow-up for durability and late effects; myeloablative conditioning toxicity and fertility implications; access, complexity, and cost constraints.
Clinical Context
Basis for FDA approval of exa-cel (Casgevy) for severe sickle cell disease (Dec 2023); first CRISPR therapy approved. Offers a one-time potentially curative option alongside allogeneic HSCT and lovo-cel gene therapy; requires myeloablative conditioning and specialized centers.