Casgevy Became the First CRISPR Gene-Editing Cure Approved by the FDA in 2023

On December 8, 2023, the U.S. Food and Drug Administration approved a therapy called Casgevy, and with that single regulatory decision, a decades-old promise made by molecular biologists finally became real. Casgevy, known generically as exagamglogene autotemcel (exa-cel), is the first medicine ever approved that uses CRISPR-Cas9 gene editing to treat a disease. It targets sickle cell disease, and for many patients it does not merely manage symptoms. It ends them.

Sickle cell disease affects approximately 100,000 Americans and more than 7 million people worldwide. The condition is caused by a single-point mutation in the gene encoding the beta subunit of adult hemoglobin, producing a distorted, crescent-shaped red blood cell that clumps, obstructs blood vessels, and starves tissues of oxygen. The result is episodes of excruciating pain called vaso-occlusive crises, along with strokes, organ damage, and a shortened life expectancy. For most patients, the only previous curative option was a bone marrow transplant from a matched donor, a procedure unavailable to the vast majority of those who need it.

How Casgevy Works

Casgevy takes a different route entirely. It does not require a donor. Instead, physicians harvest the patient's own hematopoietic stem cells, the blood-forming cells that live in bone marrow, and send them to a manufacturing facility. There, CRISPR-Cas9 is deployed to make a precise cut in a regulatory region called the BCL11A enhancer. BCL11A is a protein that acts as a repressor, switching off the gene for fetal hemoglobin (HbF) shortly after birth. In healthy adults, this shutdown is permanent. In patients with sickle cell disease, the consequences of that shutdown are devastating, because fetal hemoglobin does not sickle.

By disabling the BCL11A enhancer, Casgevy releases the brake. The edited stem cells begin producing fetal hemoglobin again, at levels high enough to dilute and functionally replace the defective adult hemoglobin. Before the edited cells are reinfused, the patient undergoes a chemotherapy conditioning regimen to clear space in the bone marrow. Once the edited cells engraft, they become a permanent, self-renewing source of fetal hemoglobin for the rest of the patient's life.

• Approval date: December 8, 2023 (FDA)
• Manufacturers: Vertex Pharmaceuticals and CRISPR Therapeutics
• Editing mechanism: CRISPR-Cas9 disruption of the BCL11A erythroid enhancer
• Cell source: Patient's own mobilized hematopoietic stem and progenitor cells (ex vivo editing)
• Phase 3 trial result: 28 of 29 patients (96.6%) were free of severe vaso-occlusive crises for at least 12 consecutive months
• Price: $2.2 million per patient (one-time treatment)

The Clinical Evidence

The Phase 3 trial results, published in the New England Journal of Medicine, were striking by any standard. In the CLIMB SCD-121 trial, 29 patients with severe sickle cell disease received Casgevy. Of those, 28, 96.6 percent, experienced no severe vaso-occlusive crises for a minimum of 12 consecutive months following treatment. The single patient who did not meet that threshold still showed a significant reduction in crisis frequency. Median follow-up at the time of the primary analysis was approximately 19 months, and the protective effect showed no signs of waning. Fetal hemoglobin levels in treated patients reached a median of around 40 percent of total hemoglobin, well above the threshold believed necessary to prevent sickling.

For context: many patients enrolled in the trial had experienced an average of three to four severe crises per year before treatment. Some had been hospitalized dozens of times. The shift to zero crises represents a transformation in daily life that goes far beyond a laboratory number.

Why It Is Historic

The FDA's approval is historic for reasons that extend beyond sickle cell disease. Casgevy is the first regulatory approval anywhere in the world for a CRISPR-based medicine. CRISPR-Cas9 was first adapted as a gene-editing tool in 2012 by Jennifer Doudna, Emmanuelle Charpentier, and their colleagues, work that earned the 2020 Nobel Prize in Chemistry. The gap between that discovery and an approved human therapy is just eleven years, extraordinarily fast by the standards of drug development.

The therapy is not without complications. The ex vivo process is lengthy, manufacturing alone takes several months, and the required conditioning chemotherapy carries real risks, including temporary infertility. The $2.2 million price tag places it among the most expensive medicines ever approved. But the scientific question, can CRISPR safely and durably edit human cells to cure a genetic disease, has now been answered. Every other CRISPR therapy in development inherits that proof as a foundation.