Casgevy: The First CRISPR Gene Therapy Gets FDA Approval

On December 8, 2023, the U.S. Food and Drug Administration approved Casgevy (exagamglogene autotemcel), making it the first therapy based on CRISPR gene editing to receive regulatory clearance anywhere in the world. Developed by Vertex Pharmaceuticals and CRISPR Therapeutics, Casgevy treats sickle cell disease (SCD) in patients aged 12 and older who experience recurrent vaso-occlusive crises. The approval marked a watershed moment for genomic medicine.

Understanding Sickle Cell Disease

Sickle cell disease affects approximately 100,000 people in the United States and millions worldwide, disproportionately impacting people of African, Mediterranean, Middle Eastern, and Indian descent. The disease is caused by a single mutation in the HBB gene, which encodes the beta-globin subunit of hemoglobin. This mutation produces abnormal hemoglobin S, which causes red blood cells to deform into a rigid, sickle-like shape.

These sickle-shaped cells block small blood vessels, triggering episodes of excruciating pain known as vaso-occlusive crises. Over time, the disease damages organs, increases stroke risk, and significantly shortens life expectancy. Before Casgevy, the only curative option was a bone marrow transplant from a matched sibling donor, an option available to fewer than 20% of patients.

How Casgevy Works

Casgevy uses an ex vivo approach, meaning the patient's own cells are edited outside the body and then returned. The process works in several steps:

1. Cell collection: Stem cells are harvested from the patient's bone marrow or mobilized into the bloodstream and collected via apheresis.
2. CRISPR editing: In the laboratory, CRISPR-Cas9 is used to make a precise cut in the BCL11A gene within the collected stem cells. BCL11A acts as a molecular switch that silences fetal hemoglobin (HbF) production after birth.
3. Myeloablative conditioning: The patient undergoes chemotherapy with busulfan to destroy their existing bone marrow, making room for the edited cells.
4. Infusion: The edited stem cells are infused back into the patient, where they engraft in the bone marrow.
5. Recovery: Over the following weeks and months, the edited cells begin producing red blood cells with high levels of fetal hemoglobin, which does not sickle.

The scientific insight behind Casgevy is elegant. Rather than trying to correct the sickle cell mutation itself, the therapy reactivates an alternative form of hemoglobin that humans naturally produce during fetal development. Fetal hemoglobin works just as well as normal adult hemoglobin for carrying oxygen, and its presence prevents the sickling process.

Clinical Trial Results

The pivotal clinical trial data that supported FDA approval were striking. In the trial, 29 out of 31 evaluable patients (93.5%) were free from vaso-occlusive crises for at least 12 consecutive months following treatment. For patients who had previously experienced an average of several crises per year, this represented a dramatic transformation in quality of life.

Patients also showed sustained high levels of fetal hemoglobin, with most achieving HbF levels above 40% of total hemoglobin. Normal adults typically have HbF levels below 1%. These elevated levels were maintained throughout the follow-up period, suggesting durability of the treatment effect.

Victoria Gray: The Face of a Revolution

Victoria Gray, a mother from Mississippi, became the first person in the United States to receive CRISPR gene editing therapy for sickle cell disease in 2019 as part of the clinical trial. Her story put a human face on what had been an abstract scientific concept.

Before treatment, Gray endured frequent pain crises that sent her to the emergency room and kept her from her family. In the years following her treatment, she reported being essentially free of sickle cell symptoms. Her testimony before Congress and her public advocacy helped raise awareness of both the promise and the access challenges surrounding gene therapy.

Gray's experience also demonstrated the difficulty of the treatment process. The myeloablative conditioning required weeks of hospitalization, and recovery took months. This is not a simple injection but a grueling medical journey, making the conversation about patient access and support all the more important.

The Cost Question

Casgevy carries a list price of approximately $2.2 million for a one-time treatment. Vertex Pharmaceuticals has argued that this price reflects the curative potential of the therapy and compares favorably to the lifetime cost of managing sickle cell disease, which has been estimated at $1.6 million to $6 million per patient over a lifetime.

However, the price has sparked intense debate. Several key challenges have emerged:

• Insurance coverage: Many patients with sickle cell disease are covered by Medicaid, and state Medicaid programs face significant budget pressures in covering high-cost gene therapies.
• Outcomes-based agreements: Vertex has pursued value-based contracts where payment is tied to treatment success, helping to share risk with payers.
• Access equity: There are concerns that the therapy may be most accessible to patients at major academic medical centers, potentially leaving rural and underserved communities behind.
• Global access: In low- and middle-income countries where sickle cell disease is most prevalent, particularly in sub-Saharan Africa, the current treatment model is economically unfeasible.

The Treatment Journey

Receiving Casgevy is not a simple outpatient procedure. The full treatment process spans several months:

• Pre-treatment evaluation and cell mobilization take several weeks.
• Cell collection and manufacturing require approximately four months.
• Myeloablative conditioning involves intensive chemotherapy over several days.
• Hospital stay during engraftment and recovery lasts several weeks.
• Follow-up monitoring continues for years.

Patients must be treated at qualified treatment centers with experience in stem cell transplantation. As of 2026, the number of authorized treatment centers continues to expand, but access remains geographically limited.

A Second Approval: Beta-Thalassemia

Shortly after its sickle cell approval, Casgevy also received FDA approval for transfusion-dependent beta-thalassemia (TDT), another hemoglobin disorder. In TDT, patients require regular blood transfusions, sometimes every two to four weeks, to survive. The same BCL11A editing strategy works for both conditions because reactivating fetal hemoglobin addresses the underlying hemoglobin deficiency in TDT as well.

Clinical results showed that 28 out of 32 evaluable TDT patients achieved transfusion independence for at least 12 months, a life-changing outcome for individuals who had been dependent on transfusions since childhood.

What Casgevy Means for the Field

The approval of Casgevy established several important precedents:

• Regulatory confidence: The FDA and EMA (which approved Casgevy even earlier, in November 2023) demonstrated that CRISPR-based therapies can clear rigorous regulatory review.
• Commercial viability: Gene therapy companies now have a real-world example of CRISPR commercialization to learn from.
• Scientific validation: The BCL11A editing strategy validated years of basic research into hemoglobin switching.
• Patient advocacy: The sickle cell community played a critical role in pushing for accelerated development and access.

Casgevy is not the end of the story. It is the beginning. Dozens of CRISPR-based therapies are in clinical trials for conditions ranging from cancer to inherited blindness to cardiovascular disease. Each will build on the lessons learned from this first approval.

For the millions of people living with sickle cell disease and other genetic conditions, Casgevy represents proof that gene editing can move from laboratory science to real-world medicine. The challenge now is making that promise accessible to everyone who needs it.

Sources & Further Reading

• FDA Approval Announcement, December 8, 2023.
• Frangoul, H. et al. "CRISPR-Cas9 gene editing for sickle cell disease and beta-thalassemia." New England Journal of Medicine 384, 252–260 (2021). — Pivotal clinical trial results.
• Victoria Gray interview, The CRISPR Journal (2024) — "Warrior Spirit: An Interview with Victoria Gray, Sickle Cell Pioneer."
• CRISPR Therapeutics Q3 2025 Earnings — 64 patients treated in 2025, 3x increase over 2024.

Last updated: March 2026.