Casgevy

Casgevy (exagamglogene autotemcel) is a one-time cell therapy in which a patient's own blood-forming stem cells are edited outside the body with CRISPR–Cas9 and returned, raising their production of fetal haemoglobin enough to compensate for a defective adult haemoglobin gene. Authorised in the United Kingdom in November 2023 and in the United States across December 2023 and January 2024, it is the first medicine based on CRISPR editing to reach regulatory approval anywhere. It treats two inherited disorders of the beta-globin gene: sickle cell disease and transfusion-dependent beta thalassaemia.

§How it works

Casgevy does not repair the mutation that causes either disease. It works around it.

Human beings make two different beta-like globin chains over a lifetime. Fetal haemoglobin, containing gamma-globin, dominates before birth and is switched off in the months afterwards, when adult beta-globin takes over. People who carry a beta-globin mutation are therefore healthy in utero and become ill only once the switch completes. A minority of people carry variants that keep fetal haemoglobin switched on into adulthood — hereditary persistence of fetal haemoglobin — and if they also inherit sickle mutations, their disease is markedly milder. That natural experiment defined the therapeutic target.

The switch is enforced by the transcription factor BCL11A, identified as the repressor of fetal haemoglobin by Stuart Orkin's group in 2008.¹ Knocking out BCL11A entirely is unacceptable — it is needed in B cells and in the brain — but the gene carries an enhancer, active only in the erythroid lineage, whose disruption lowers BCL11A in red cell precursors and nowhere else.² A saturating mutagenesis screen mapped the critical bases within that enhancer, giving a small target that a single guide RNA can hit.³

The manufacturing sequence is that of a stem cell transplant with an editing step inserted. Stem cells are mobilised from the marrow with plerixafor and collected by apheresis, often over several cycles. In the factory, CD34+ cells are electroporated with a Cas9 ribonucleoprotein complex carrying the guide; the protein enters as a preassembled complex rather than being encoded on a viral genome, so it acts within hours and is then degraded. Nothing persists in the cell, which is why no viral vector is involved and why immunity to a capsid is not a consideration. The edited cells are frozen and tested. The patient then receives myeloablative busulfan to empty the marrow niche, and the edited cells are infused.

The edit itself is a blunt one — a double-strand break repaired by error-prone end joining, producing small insertions and deletions that happen to destroy an enhancer motif. Competing programmes aim at the same biology with gentler chemistry: Base editing can install the exact single-base changes found in people with naturally persistent fetal haemoglobin, and Epigenome editing could in principle silence the enhancer without breaking DNA at all. Correcting the sickle mutation directly, which Prime editing makes conceivable, would be the more complete solution and is further from the clinic.

§Development history

1. 2008
   BCL11A identified
   Orkin's laboratory shows that BCL11A is the developmental-stage repressor that silences fetal haemoglobin after birth.
2. 2013–2015
   The enhancer as a target
   The erythroid-specific BCL11A enhancer is characterised and then dissected base by base with Cas9, defining a narrow, lineage-restricted target.
3. 2019
   First patients dosed
   Victoria Gray in the United States and a beta thalassaemia patient in Germany receive edited cells in the CLIMB trials.
4. 2021
   First results published
   The New England Journal of Medicine reports elimination of vaso-occlusive crises and transfusion independence in the first treated patients.
5. 2023
   Approval
   The UK MHRA authorises the therapy in November; the FDA follows in December for sickle cell disease, and in January 2024 for beta thalassaemia.
6. 2024–2025
   Reimbursement fights
   European and UK health-technology bodies negotiate managed-access arrangements; uptake remains slow and confined to authorised transplant centres.

§Clinical results

In the pivotal single-arm trials, nearly all evaluable sickle cell patients went at least twelve consecutive months without a severe vaso-occlusive crisis, the painful episodes of blocked microcirculation that define the disease and drive its mortality.⁴ Most thalassaemia patients became independent of the regular red cell transfusions they had needed since childhood.⁵ Fetal haemoglobin typically rose to a large fraction of total haemoglobin and stayed there, and edited alleles persisted in the marrow, indicating that true long-term stem cells had been modified rather than short-lived progenitors.

Two caveats belong beside those numbers. The trials had no control arm, so the comparison is with each patient's own history rather than with a randomised alternative. And follow-up is short relative to the claim being made. A therapy sold as a cure needs decades of surveillance to establish that it is one, particularly given a theoretical risk of clonal expansion from an edited stem cell.

What "cure" means here

The causative mutation is untouched. Every red cell a treated patient makes still carries it, and any child they have inherits it exactly as before. Casgevy suppresses the phenotype by reactivating a fetal gene; it does not correct the genome's error, and it changes nothing about inheritance in the way Human germline editing would.

§The conditioning burden

The editing is the easy part. The hard part is busulfan.

To make room for edited cells, the existing marrow must be destroyed with high-dose chemotherapy. That carries weeks of neutropenia and infection risk, mucositis, hospitalisation typically measured in a month or more, a high probability of permanent infertility, and a small long-term risk of secondary malignancy. Patients are advised to bank gametes beforehand, which links the therapy to the reproductive questions covered under Embryo selection and fertility preservation. For a young adult with severe disease the trade may be worth it; for a person with moderate disease it plainly is not, and this is why eligibility is drawn narrowly.

Removing conditioning is the field's main objective. Antibody-based marrow clearance and, further out, editing haematopoietic stem cells directly inside the body using Lipid nanoparticles targeted to CD117-bearing cells would eliminate the chemotherapy step entirely. Both have shown promise in animals. Neither is an approved treatment as of 2026.

§Price and access

The US list price is about $2.2 million, alongside roughly $3.1 million for Lyfgenia, the lentiviral therapy approved the same day. Total cost of care — mobilisation, apheresis, conditioning, inpatient stay, and follow-up — is higher still. England's health-technology assessor initially declined to recommend the therapy on cost-effectiveness grounds and later agreed managed-access arrangements, first for beta thalassaemia and then for sickle cell disease.

The mismatch

Roughly 100,000 people in the United States live with sickle cell disease. Global prevalence is in the millions, and the overwhelming majority of affected births occur in sub-Saharan Africa and India, where apheresis and transplant infrastructure is scarce or absent.⁶

Uptake in the first two years after approval was slow. Patients must travel to an authorised centre, undergo mobilisation that can fail, and accept myeloablation; payers must find the money; and each course consumes scarce transplant-unit capacity. The result is a therapy that works and that almost nobody receives — an unusually stark instance of the pattern described in Access and inequality and Somatic gene therapy more generally. It has also sharpened arguments about the governance of who benefits from publicly funded discovery, since the biology underlying the target was worked out largely in academic laboratories.

§Outlook

Casgevy's importance is partly symbolic. It established that a CRISPR-edited cell product can pass a regulator's benefit-risk review, and it made the off-target analysis package for an editing therapy a known, negotiable quantity rather than an open question. Five years after the He Jiankui affair had made CRISPR in humans synonymous with recklessness, the approval demonstrated what the ordinary route through regulatory review looks like when it is followed. What follows depends less on editing chemistry than on delivery and logistics. If in vivo editing of blood stem cells works in humans, the same biological target could in principle be addressed with an injection in a district hospital. If it does not, the therapy remains a very expensive procedure available in a few dozen buildings worldwide, and the disease it treats will continue to be managed, for almost everyone who has it, with hydroxyurea and transfusion.

§See also

• CRISPR–Cas9
• Somatic gene therapy
• Base editing
• Lipid nanoparticles
• Human germline editing
• Access and inequality
• Off-target effects in genome editing
• Governance of human genome editing

§References