By David | Biotech News Today
Sickle cell disease gene therapy is a one-time treatment that changes a patient’s own blood stem cells so they make healthier red blood cells. In December 2023, the U.S. FDA approved two such therapies, Casgevy and Lyfgenia, for patients aged 12 and older who have had repeated episodes of severe pain. Both are designed to reduce or stop these pain crises, but they use different methods and carry different risks.
This article explains what sickle cell disease is, how gene therapy for sickle cell works, how the two approved products compare, and what patients should know about the process, the risks and the cost.
What Is Sickle Cell Disease?
Sickle cell disease is an inherited blood disorder caused by a change (mutation) in the HBB gene. This gene gives the body instructions for making part of hemoglobin, the protein in red blood cells that carries oxygen. In people with sickle cell disease, the faulty gene produces an abnormal hemoglobin called hemoglobin S.
Normal red blood cells are round and flexible. Cells with hemoglobin S can turn stiff and curved, like a sickle. These cells can block small blood vessels and break down early. This leads to:
- Sudden, severe pain episodes, often called pain crises or vaso-occlusive events
- Long-term anemia and tiredness
- A higher risk of infections and stroke
- Gradual damage to organs such as the kidneys, lungs and spleen
The U.S. Centers for Disease Control and Prevention estimates that about 100,000 people in the United States live with the condition, and it is also common in parts of Africa, the Middle East and India.
Why Gene Therapy for Sickle Cell?
Until recently, treatment focused on managing symptoms and complications. Medicines such as hydroxyurea and regular blood transfusions help many patients but do not remove the underlying cause. A bone marrow transplant from a matched donor can cure the disease, but many patients do not have a suitable donor, and the procedure carries its own risks.
Gene therapy offers another route. Because it uses the patient’s own cells, no donor is needed. The basic science behind it is explained in our guide on how gene therapy works.
How Gene Therapy for Sickle Cell Works
Both approved therapies are ex vivo, which means the cells are changed in a laboratory and then returned to the patient. The process follows these main steps:
- Collecting stem cells: Blood stem cells are taken from the patient’s blood through a process called apheresis. Several collection sessions may be needed.
- Modifying the cells: In a specialized manufacturing facility, the cells are either edited or given a new gene. Making these products is complex work, and companies often rely on a CDMO for part of it.
- Preparing the body: Before the modified cells are returned, the patient receives strong chemotherapy (conditioning) to clear space in the bone marrow.
- Infusion: The modified stem cells are infused back into the patient through a vein.
- Recovery and monitoring: The cells settle in the bone marrow and begin producing new red blood cells. Patients usually stay in hospital for several weeks and are followed for years afterwards.
The whole journey, from cell collection to infusion, can take many months because manufacturing takes time.
Approved Sickle Cell Gene Therapies: Casgevy vs. Lyfgenia
| Feature | Casgevy | Lyfgenia |
|---|---|---|
| Generic name | exagamglogene autotemcel | lovotibeglogene autotemcel |
| Developer | Vertex Pharmaceuticals and CRISPR Therapeutics | bluebird bio |
| Method | CRISPR-Cas9 gene editing | Gene addition using a lentiviral vector |
| What it does | Edits a gene switch so the body makes more fetal hemoglobin, which does not sickle | Adds a modified gene that makes an anti-sickling form of hemoglobin |
| FDA approval | December 2023 | December 2023 |
| Patient group | Ages 12+ with recurrent severe pain crises | Ages 12+ with a history of severe pain crises |
| Notable safety note | Low blood counts and conditioning-related risks | Boxed warning for blood cancer (hematologic malignancy) |
| List price at launch | About $2.2 million | About $3.1 million |
Casgevy is notable because it is the first FDA-approved therapy based on CRISPR gene editing. Instead of fixing the faulty gene, it targets a different gene, called BCL11A, that normally switches off fetal hemoglobin after birth. Turning that switch down lets the body produce fetal hemoglobin again, and this form of hemoglobin does not cause cells to sickle.
Lyfgenia takes the gene addition route. It uses a modified virus to give the stem cells a working version of a beta-globin gene. The cells then produce a hemoglobin that resists sickling.
How Does Gene Therapy Compare With Other Treatments?
| Option | What it does | Main limits |
|---|---|---|
| Hydroxyurea | Daily medicine that increases fetal hemoglobin and lowers pain episodes | Needs lifelong use and monitoring; does not work equally well for everyone |
| Blood transfusions | Replace sickled cells with healthy donor cells | Repeated transfusions can cause iron overload and other complications |
| Bone marrow transplant | Can cure the disease | Needs a matched donor and carries risks such as graft-versus-host disease |
| Gene therapy | One-time treatment using the patient’s own cells | Intensive process, high cost, limited treatment centers, long-term data still growing |
Risks and Side Effects
Gene therapy for sickle cell is a major medical procedure, and patients should discuss the risks with a specialist team.
- Conditioning chemotherapy: This step causes low blood counts, a higher risk of infection, mouth sores and nausea. It can also affect fertility, so doctors usually discuss fertility preservation before treatment.
- Long hospital stay: Patients stay in hospital until their blood counts recover.
- Blood cancer risk with Lyfgenia: The FDA added a boxed warning because of cases of blood cancer reported with this product, so long-term monitoring is required.
- Uncertain long-term results: These therapies are new, and doctors are still collecting data on how long the benefits last.
Cost and Access
At launch, Casgevy had a list price of about $2.2 million and Lyfgenia about $3.1 million. Treatment is available only at specialized centers, and the process of insurance approval can take time. Governments and insurers are testing new payment models, but access remains a major challenge, especially in lower-income countries where sickle cell disease is most common. These products are part of the wider field of biologics and advanced therapies.
Frequently Asked Questions
Can gene therapy cure sickle cell disease?
Gene therapy is designed to give long-lasting benefit, and many patients in clinical trials had no severe pain crises after treatment. However, these therapies are still new, so doctors are careful about using the word “cure” until more long-term data is available.
Which gene therapy is approved for sickle cell disease?
Two are approved in the United States: Casgevy and Lyfgenia. Both were approved by the FDA in December 2023 for patients aged 12 and older.
Is CRISPR used for sickle cell disease?
Yes. Casgevy uses CRISPR-Cas9 to edit a patient’s own blood stem cells so they produce more fetal hemoglobin.
How long does sickle cell gene therapy take?
The full process can take many months, including stem cell collection, manufacturing, conditioning and recovery in hospital.
Who can get sickle cell gene therapy?
Eligibility depends on age, the severity of the disease and overall health. A hematology specialist at a treatment center decides whether a patient is a suitable candidate.
Key Takeaways
- Sickle cell disease is caused by a mutation in the HBB gene, which leads to sickled red blood cells and painful crises.
- Casgevy (gene editing) and Lyfgenia (gene addition) were both approved by the FDA in December 2023.
- Both therapies use the patient’s own stem cells and need chemotherapy before the cells are returned.
- Cost, access and long-term safety remain the main challenges.
This article is for general information only and is not medical advice. Patients should speak to their doctor about treatment options.
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