Gene Therapy Drugs
Gene therapy aims to treat disease by modifying the genetic material inside a patient's cells. Learn how approved gene therapies work and what makes them different from conventional drugs.
What Is Gene Therapy?
Gene therapy treats disease by altering the genetic material in a patient's cells. Rather than compensating for a missing or broken protein with an external drug, gene therapy attempts to fix the underlying genetic problem itself — either by adding a functional copy of a gene, silencing a disease-causing gene, or editing the existing DNA sequence.
The concept dates to the 1970s, but clinical translation took decades because of delivery challenges and safety setbacks. The field accelerated significantly in the 2010s, and by the mid-2020s, more than 30 gene therapies had received regulatory approval worldwide.
Types of Gene Therapy
Gene addition (augmentation): A working copy of a gene is delivered to cells that carry a faulty or missing version. The new gene does not replace the old one; it coexists and produces the functional protein the body was lacking. This is the most common approach for recessive disorders where having at least one working copy is sufficient.
Gene silencing: Certain diseases are caused by overactive or mutant genes that produce harmful proteins. RNA interference (RNAi) therapies use small RNA molecules to degrade the mRNA from these genes before the harmful protein can be made. Inclisiran, approved for high cholesterol, uses this mechanism.
Gene editing: Tools like CRISPR-Cas9 can cut and modify specific DNA sequences. Unlike gene addition, editing can correct the original mutation. Casgevy, approved in 2023 for sickle cell disease and beta-thalassemia, is the first approved CRISPR-based therapy.
Ex vivo vs. in vivo: Ex vivo therapies remove cells from the patient, modify them in the laboratory, and reinfuse them. In vivo therapies deliver the genetic modification directly into the patient's body.
Delivery Systems: Vectors
Getting genetic material into cells requires a delivery vehicle called a vector. Most approved gene therapies use modified viruses, stripped of their disease-causing components but retaining their ability to enter cells.
- Adeno-associated virus (AAV): Small, non-integrating virus preferred for in vivo therapies targeting the eye, liver, muscle, and nervous system. Different AAV serotypes have affinity for different tissues (tropism). A key limitation is pre-existing immunity — many people have antibodies against common AAV serotypes from prior natural exposure.
- Lentivirus: Integrates into the genome, enabling long-term expression. Used mainly in ex vivo therapies modifying stem cells or T cells.
- Non-viral vectors: Lipid nanoparticles (same as mRNA vaccines), electroporation (electrical pulses to open cell membranes), and nanoparticles are being developed to avoid immune responses to viral vectors.
Approved Gene Therapies
Several landmark gene therapies are now in clinical use:
- Luxturna (voretigene neparvovec): AAV gene therapy for a rare inherited retinal dystrophy caused by RPE65 mutations. A single injection per eye can restore functional vision.
- Zolgensma (onasemnogene abeparvovec): One-time AAV therapy for spinal muscular atrophy (SMA) type 1 in children under two. Delivers a functional SMN1 gene to motor neurons.
- Hemgenix (etranacogene dezaparvovec): AAV therapy for hemophilia B. A single infusion produces sustained factor IX levels in most patients.
- Casgevy (exagamglogene autotemcel): First approved CRISPR therapy, for sickle cell disease and transfusion-dependent beta-thalassemia.
- Lyfgenia (lovotibeglogene autotemcel): Lentiviral gene therapy also approved for sickle cell disease.
Orphan Drugs and Gene Therapy
Gene therapy is closely linked to orphan drugOrphan Drug A drug developed to treat a rare disease or condition affecting fewer than 200,000 people in the United States. The Orphan Drug Act provides manufacturers with incentives including tax credits, grant
Nearly all approved gene therapies target orphan conditions. The small patient populations mean clinical trials can be smaller, and regulators often accept surrogate endpoints (like factor IX levels rather than bleeding episodes) to speed approval.
Risks and Safety History
The field's history includes serious setbacks. In 1999, Jesse Gelsinger died from a massive immune response to an adenoviral vector in a gene therapy trial — a watershed event that halted the field for years. In early lentiviral trials for X-linked SCID ("bubble boy" disease), several children developed leukemia when the viral vector inserted near cancer-promoting genes.
Modern therapies have benefited from safer vector designs, immunosuppression protocols, and better patient selection. Serious adverse events still occur, particularly immune-mediated reactions to high-dose AAV therapies, and several patients have died in high-dose AAV trials for Duchenne muscular dystrophy. Ongoing surveillance is essential.
Cost and Access
Gene therapies are among the most expensive treatments ever developed. Zolgensma costs approximately $2.1 million per dose. Hemgenix was priced at $3.5 million at launch — the highest price for any approved drug at the time.
Manufacturers argue that a one-time cure justifies high prices through health economics modeling comparing lifetime drug costs. Payers have pushed back, and novel payment models — including installment payments and outcomes-based contracts — are being explored. Access in lower-income countries remains severely limited.
Key Takeaways
- Gene therapy modifies genetic material to treat disease at its source, using gene addition, silencing, or editing.
- AAV vectors are the most common delivery system for in vivo therapies; lentiviruses dominate ex vivo approaches.
- More than 30 gene therapies are now approved globally, mostly for rare inherited disorders.
- Most approved gene therapies carry orphan drug designation, reflecting their rare-disease focus.
- Costs are extremely high; innovative payment and access models are developing alongside the science.