Viral Vector-Based Gene Therapies: How Engineered Viruses Are Treating Disease

Table of Contents

• Key Points
• Background: What Is Gene Therapy and Why Do We Need Viral Vectors?
• The Viral Vectors Used in the Clinic
• Approved Gene Therapies for Cancer
• Approved Gene Therapies for Vaccination (Ebola and COVID-19)
• Approved Gene Therapies for Eye, Neurological, and Metabolic Diseases
• The Current Clinical Trial Landscape
• What These Approvals Mean for Patients
• Challenges in Viral Vector Gene Therapy
• Recommendations for Patients and Families
• Frequently Asked Questions
• Source Information

Key Points

• Gene therapy uses engineered viruses to deliver corrective genes, changing treatment for some cancers, inherited diseases, and infections.
• Thirteen in vivo viral vector gene therapies are approved worldwide, including for melanoma, head and neck cancer, SMA, and RPE65 blindness.
• AAV vectors offer long-term low-immunogenicity expression; adenovirus and herpes simplex virus are used mostly for cancer and vaccines.
• Cancer therapies showed improved responses: IMLYGIC improved melanoma survival from 18.9 to 23.3 months; Gendicine achieved a 90% response rate.
• Challenges include immune reactions, insertional mutagenesis, limited gene cargo, and transient expression; long-term safety data are still being collected.

Background: What Is Gene Therapy and Why Do We Need Viral Vectors?

Gene therapy is a treatment approach that modifies the expression of genes or corrects dysfunctional genes. Unlike traditional drugs, which usually work by blocking or activating proteins, gene therapy genetically modifies cells. This opens the possibility of curing diseases once thought to be incurable.

The concept dates back to the 1960s, when early studies showed that DNA sequences could be introduced into mammalian cells for gene repair. Decades of work led to the first human gene therapy clinical trial in 1990, which used a retrovirus vector to treat severe combined immunodeficiency (SCID), a condition in which children are born without a functioning immune system.

Gene therapy suffered a major setback in the late 1990s. In 1999, a patient died in a clinical trial due to severe immune responses triggered by the viral vector. In 2000, four patients developed leukemia after receiving a retrovirus-based gene therapy. These two events halted gene therapy in the clinic, raised serious safety concerns, and highlighted the urgent need for safer viral vectors.

The following decade focused on understanding viral biology and engineering safer, more effective vectors. This led to the first regulatory approvals. China approved the world's first gene therapy product, Gendicine®, for head and neck cancer in 2003. The European Medicines Agency (EMA) approved its first gene therapy product, Glybera®, in 2012. The United States approved its first, Kymriah®, in 2017.

With the development of gene editing technologies such as CRISPR/Cas9, which can precisely modify genes at the level of individual DNA bases, gene therapy is entering a new era and rapidly expanding to treat a broader spectrum of diseases.

Current gene therapies fall into two broad categories: ex vivo and in vivo. Ex vivo therapies involve removing cells from a patient, modifying them in the laboratory, and returning them to the body. In vivo therapies involve directly infusing gene therapeutics into the patient's bloodstream or injecting them into target organs. This review focuses on in vivo therapies, which generally require a vector to package and deliver the gene material into target cells. Engineered viruses are the dominant vectors in current gene therapy clinical studies.

The Viral Vectors Used in the Clinic

The primary reason viruses are used as delivery vehicles is their natural ability to infect cells. Researchers exploit this ability to shuttle gene material of interest into host cells. The major viral vector types used for in vivo gene therapies include adenovirus (Ad), adeno-associated virus (AAV), herpes simplex virus (HSV), retrovirus, and lentivirus. Each has distinct characteristics, advantages, and limitations.

Adenovirus (Ad)

Adenovirus was one of the earliest viral vectors studied in the clinic. Ads are a class of DNA viruses with a 34–43 kb genome enclosed in a nonenveloped icosahedral (20-sided) viral particle. There are more than 50 Ad serotypes, with Ad5 and Ad26 being the most widely used for gene therapy.

Three generations of Ad vectors have been engineered for different therapeutic applications:

1. First generation: Removal of the E1 and E3 gene units, making the vector capable of carrying up to 7.5 kb of foreign DNA.
2. Second generation: Deletion of E1 and E4 units, which significantly reduces immunogenicity (the ability to trigger an immune response).
3. Third generation: Deletion of all viral genes, allowing the vector to carry more than 30 kb of foreign DNA.

Notable characteristics of Ad vectors include:

• Multiple genome copies can be delivered into one host cell, leading to high gene expression levels.
• Gene expression is transient because the DNA cargo stays episomal (separate from the host genome) and is not integrated.
• They can transduce both dividing and nondividing cells.
• They are highly immunogenic.

Because of these characteristics, Ad vector-based gene therapies are primarily used for treating cancer and for vaccination against infectious diseases.

Adeno-Associated Virus (AAV)

AAV is the most widely used viral vector for in vivo gene therapy applications. AAV is a nonpathogenic parvovirus (a type of small virus that does not cause disease in humans) with a 4.7 kb DNA genome enclosed in a nonenveloped icosahedral capsid. AAV has 11 natural serotypes and over 100 variants. Different serotypes have tropism (preferential targeting) toward different tissues, making each serotype suitable for gene delivery to specific organs.

For example, AAV9 shows tropism toward central nervous system (CNS, the brain and spinal cord) organs, while AAV8 can effectively transduce the pancreas.

Major characteristics of AAV vectors include:

• They can transduce both dividing and nondividing cells and do not integrate DNA into the host genome.
• They enable long-term, stable gene expression.
• They have low immunogenicity.

Because of these unique features, AAV is the most suitable viral vector for in vivo gene therapies, especially for conditions requiring long-term gene modification. Their primary clinical applications cover a broad array of monogenic diseases (diseases caused by a single gene mutation), including ophthalmological (eye) diseases, metabolic diseases, hematological (blood) diseases, neurological diseases, and musculoskeletal diseases.

Herpes Simplex Virus (HSV)

HSV is an enveloped virus with a double-stranded DNA genome of over 150 kb. The virus genome encodes approximately 90 genes; half of these are nonessential and can be removed or replaced in recombinant vectors, affording a high capacity for foreign DNA. Eight human HSV serotypes have been identified, and each exhibits distinct tropism.

Three major types of HSV vectors have been engineered for gene therapy:

1. Amplicon HSV: An engineered vector that can carry a large foreign DNA payload of over 100 kb.
2. Replication-defective HSV: Created by depleting genes necessary for the lytic (cell-destroying) cycle of HSV; less toxic and less immunogenic.
3. Replication-competent HSV: Keeps genes for replication in the laboratory but deletes genes needed for replication inside the human body.

Major characteristics of HSV vectors include their ability to evade the immune system, their capacity to deliver large DNA cargos and multiple genes, and their intrinsic or engineered cell-specific lytic property. Clinical application of HSV vector-based gene therapies has primarily focused on cancer treatment, mostly attributed to their intrinsic oncolytic (cancer-killing) capability.

Retrovirus

Retrovirus was the first viral vector studied in clinical trials for in vivo gene therapy. Retrovirus is an enveloped spherical virus that carries its genetic material in the form of RNA. Retrovirus vectors can reverse transcribe their genetic material (single-stranded RNA) into double-stranded DNA and integrate it into the host cell's genome.

The major advantages of retrovirus vectors are that they can carry a large gene of interest (9–12 kb) and result in long-term gene expression due to integration into the host genome. However, several major drawbacks limit their application:

• Retrovirus vectors require cell division to integrate their DNA into the host genome, so they can only transduce dividing cells.
• They carry the risk of randomly inserting DNA into the host chromosome, leading to insertional mutagenesis (mutations caused by DNA insertion).

Self-inactivating vectors, which have the promoter or enhancer of the long terminal repeat deleted, have been developed to reduce the risk of insertional mutagenesis. Due to these limitations, retrovirus vectors are not often used in clinical studies anymore.

Lentivirus

Lentivirus is another important viral vector, although its major application is for ex vivo gene therapies. It is also being investigated in the clinic for in vivo applications. Lentivirus is a subtype of retrovirus and carries genetic material in the form of RNA. However, unlike retrovirus, lentivirus can integrate its genome into and transduce nondividing cells.

The first generation of lentivirus vectors was originally derived from HIV-1 (human immunodeficiency virus type 1) and has proven capable of efficiently transducing CNS organs in vivo. Newer generations are derived from nonhuman lentiviruses and are theoretically more acceptable because their parental viruses do not infect humans.

Distinguishing characteristics of lentivirus vectors include:

• Capability of transducing both dividing and nondividing cells
• Capability to enable long-term gene expression
• Reduced risk of genotoxicity and insertional mutagenesis compared with retrovirus vectors

The main disadvantage of lentivirus vectors is their limited genetic cargo capability. The primary application of lentivirus vectors for in vivo gene therapies is treating monogenic diseases and chronic diseases, including neurological, ophthalmological, and metabolic diseases.

Other Viral Vectors

Other viral vectors are also studied in the clinic for in vivo gene delivery, albeit to a lesser extent. These include vesicular stomatitis virus (VSV), modified vaccinia virus Ankara (MVA), arenavirus, Sendai virus, and measles virus. Clinical applications of these vectors are based on their unique properties. For example, VSV is studied for treating liver cancer and advanced solid tumors because of its intrinsic oncolytic capability. MVA is widely used for vaccination applications due to its tropism toward antigen-presenting cells (immune cells that display foreign proteins to trigger an immune response).

Approved Gene Therapies for Cancer

Four viral gene therapy products have been approved globally for the treatment of cancer, particularly solid tumors. Two are based on adenovirus, and the remaining two use HSV or retrovirus as the viral vector. The principal mechanisms of action are: (1) the engineered viral vectors have intrinsic oncolytic properties that kill cancer cells directly, and/or (2) the carried gene leads to expression of tumor suppressors or immunomodulators that enhance the anti-tumor immune response. Notably, after approval for a specific cancer type, all of these products have been or are being studied in the clinic for treating various additional types of cancer.

IMLYGIC® (Talimogene Laherparepvec)

IMLYGIC® is the first and only US FDA-approved viral gene therapy for cancer. It was approved in 2015 by both the US FDA and the EMA for the treatment of local recurrent unresectable cutaneous (skin), subcutaneous (under the skin), and nodal (lymph node) melanoma after initial surgery. It is administered directly into lesions (intralesional injection).

IMLYGIC® is based on a modified HSV1 in which two viral genes, γ34.5 and α47, were deleted and replaced with the human GM-CSF gene (granulocyte-macrophage colony-stimulating factor, a protein that stimulates immune cells). The deletion of the γ34.5 gene enables the virus to selectively replicate in tumors but not in normal tissues. Deletion of the α47 gene removes the virus's ability to suppress immune responses, which helps activate the immune system. Locally expressed GM-CSF attracts dendritic cells (immune cells that present antigens) to the tumor for antigen presentation, leading to an adaptive immune response against the tumor.

In the late-stage trial that led to approval, IMLYGIC® demonstrated a significantly higher response rate (16.3% versus 2.1%) and improved overall survival (23.3 months versus 18.9 months) compared with the control therapy GM-CSF alone.

Apart from melanoma, IMLYGIC® has been clinically investigated for treating other solid tumors. A search of Clinicaltrials.gov indicated that IMLYGIC® was mentioned in more than 30 active trials studying its capability to treat various cancers, including soft tissue sarcoma, triple negative breast cancer, ovarian cancer, pancreatic cancer, and rectal cancer.

Gendicine® (Ad-p53)

Gendicine® is the world's first approved cancer viral gene therapy. It was approved in China in 2003 for treating head and neck squamous cell carcinoma. Gendicine® is based on a human Ad5 in which the E1 gene was replaced by the gene encoding human wild-type p53, a well-known tumor suppressor protein.

Distinct from IMLYGIC®, the mechanism of action of Gendicine® depends on the expression of p53 protein in tumor cells. This initiates apoptotic (programmed cell death) pathways, suppresses anti-apoptotic events, and blocks survival pathways in cancer cells. Gendicine® achieved a 90% total response rate, which was significantly higher than that achieved by conventional chemotherapy alone. It is administered via intratumoral, intracavity, or intravenous routes.

Gendicine® has been clinically studied for treating various cancers, including bladder cancer, ovarian cancer, lung cancer, breast cancer, and liver cancer. It is currently mentioned in an active trial investigating its combination with immune checkpoint inhibitors (drugs that help the immune system recognize and attack cancer cells) for treating solid tumors.

Oncorine® (H101)

Oncorine® is another Ad5-based cancer viral gene therapy. It was approved in China in 2005 for the treatment of late-stage refractory nasopharyngeal cancer (cancer of the upper throat behind the nose) in combination with chemotherapy. It is an oncolytic viral gene therapy based on a modified Ad5 in which the viral gene E1B-55KD is completely deleted; no exogenous genes were incorporated into the virus.

The defect in E1B-55KD allows the virus to selectively replicate in and kill cancer cells. In clinical trials, Oncorine® combined with chemotherapy produced a significantly higher overall response rate compared with chemotherapy alone. It is administered intravenously and is also being investigated for conditions including refractory malignant ascites (fluid accumulation in the abdomen caused by cancer) and hepatocellular carcinoma (liver cancer).

Rexin-G® (Mx-dnG1)

Rexin-G® is a retrovirus-based cancer viral gene therapy carrying a cytocidal (cell-killing) cyclin G1 gene. It was approved in the Philippines in 2007 by the Bureau of Food and Drug (BFAD) for treating solid tumors, including soft tissue sarcoma, osteosarcoma, and pancreatic cancer. Rexin-G® is administered intravenously.

Unlike the other three approved cancer products, Rexin-G® is a tumor-targeting viral gene therapy. Its tumor-targeting ability comes from displaying a cryptic SIG-binding peptide on the viral vector that selectively binds to abnormal Signature (SIG) proteins present in tumors. The mechanism of action is primarily based on the expression of cyclin G1 in tumor cells, which arrests the cell cycle in the G1 phase and triggers cell death and apoptosis.

Following its approval in the Philippines, Rexin-G® has been investigated in several completed Phase 1 or Phase 2 trials in the United States for treating various cancers, including pancreatic cancer, sarcoma, breast cancer, and osteosarcoma. The US FDA granted Rexin-G® orphan drug designation (a status given to drugs for rare diseases) for osteosarcoma and soft tissue sarcoma in 2008 and fast-track designation for pancreatic cancer in 2009.

Rexin-G® is currently mentioned in two active clinical trials. Interestingly, one active trial is investigating Rexin-G® for treating COVID-19. The rationale is that it can target exposed collagenous proteins in injured lungs, enter and kill rapidly dividing T cells (immune cells), and thereby reduce cytokine release (a dangerous overreaction of the immune system) and acute respiratory distress syndrome.

Approved Gene Therapies for Vaccination (Ebola and COVID-19)

Six viral gene therapy products have been approved or authorized for emergency use for vaccination against infectious diseases: Ebola virus infections and COVID-19. All are based on recombinant replication-incompetent viral vectors (viruses engineered so they cannot replicate and cause disease) carrying the gene that encodes target virus surface proteins.

The mechanism of action depends on two steps: (1) efficient entry of the viral vector into cells at or around injection sites, delivering the gene construct into those cells, and (2) the delivered gene instructing the cells to overexpress the target virus surface protein, which stimulates the immune system to produce cellular and humoral (antibody-based) responses against the infectious virus.

Notably, all six vaccines except JNJ-78436735 and Convidicea use a two-dose schedule. In two products (Zabdeno®/Mvabea® and Sputnik V), two different viral vectors are used for the first and second doses. The rationale is that antibodies against the first viral vector generated after the first dose might neutralize and reduce the efficacy of the second dose. Using a different viral vector in the second dose bypasses this concern.

Ervebo® (rVSV-ZEBOV)

Ervebo® is the world's first Ebola virus vaccine and was approved by the EMA and US FDA in 2019. It is based on the vesicular stomatitis virus (VSV) vector and is administered intramuscularly. Vaccine efficacy was 100% in the pivotal trial. Antibody responses were seen in 90.0%–97.8% of subjects at 1 month after vaccination and in 83.2%–95.4% at 6 months after vaccination. The vaccine was well tolerated. It was approved for preventing Ebola virus infection.

Zabdeno® and Mvabea® (Ad26.ZEBOV and MVA-BN-Filo)

This two-dose Ebola vaccine regimen was approved by the EMA in 2020. The first dose (Zabdeno®) uses an Ad26 vector, and the second dose (Mvabea®) uses an MVA (modified vaccinia virus Ankara) vector. Both are administered intramuscularly. In clinical trials, 98%–100% of study participants mounted an antibody response after the two vaccine doses. The vaccine was well tolerated.

COVID-19 Vaccines

Four viral vector-based COVID-19 vaccines have been approved or authorized for emergency use:

• JNJ-78436735 (formerly Ad26.COV2.S, Johnson & Johnson): Approved or authorized for emergency use in more than 30 countries in 2021. It uses an Ad26 vector and is given as a single intramuscular dose. Overall efficacy was 66% for one-dose vaccination. It was well tolerated.
• Sputnik V (formerly Gam-COVID-Vac, Gamaleya Research Institute): Approved or authorized for emergency use in more than 60 countries in 2021. It uses two different adenovirus vectors — Ad26 for the first dose and Ad5 for the second dose — given intramuscularly. Overall efficacy was 79% for two-dose vaccination. It was well tolerated.
• Convidicea (Ad5-nCoV, CanSino Biologics): Approved in 2021 by China's NMPA and in Mexico, Pakistan, Chile, Hungary, and Moldova. It uses an Ad5 vector and is given as a single intramuscular dose. Overall efficacy was 65% for single-dose vaccination. It was well tolerated.
• AZD1222 (also known as Covishield in India, AstraZeneca/Oxford): Authorized for emergency use in more than 110 countries in 2020 and 2021. It uses a chimpanzee adenovirus (ChAd) vector and is given as two intramuscular doses. Overall efficacy was 76% for two-dose vaccination. It was well tolerated.

Approved Gene Therapies for Eye, Neurological, and Metabolic Diseases

Beyond cancer and infectious diseases, three approved products use AAV vectors to treat monogenic diseases affecting the eye, the nervous system, and metabolism.

Luxturna® (Voretigene Neparvovec) for Inherited Blindness

Luxturna® was approved by the US FDA in 2017, Health Canada in 2020, and Australia's Therapeutic Goods Administration (TGA) in 2020. It treats Leber's congenital amaurosis, an inherited retinal disease caused by biallelic mutations (mutations in both copies of a gene) in the RPE65 gene, which leads to severe vision loss in children.

Luxturna® uses an AAV2 vector and is administered by subretinal injection (injection under the retina). In the pivotal trial, patients treated with Luxturna® demonstrated significant improvement in functional vision compared with control groups, as measured by the multi-luminance mobility test (MLMT), a test that assesses the ability to navigate an obstacle course at different light levels. The score change from baseline to Year 1 was the key outcome measure.

Zolgensma® (Onasemnogene Abeparvovec) for Spinal Muscular Atrophy

Zolgensma® was approved by the US FDA in 2019, the EMA in 2020, and Japan's Ministry of Health and Welfare (JMHW) in 2020. It treats spinal muscular atrophy (SMA), a severe neuromuscular disease caused by biallelic mutations in the survival motor neuron 1 (SMN1) gene. SMA leads to progressive muscle weakness and, in its most severe forms, death in early childhood.

Zolgensma® uses an AAV9 vector and is administered as a single intravenous infusion. Patients treated with Zolgensma® demonstrated significant improvement in their ability to reach developmental motor milestones, such as head control and the ability to sit without support. It is approved for pediatric patients less than 2 years of age.

Glybera® (Alipogene Tiparvovec) for Lipoprotein Lipase Deficiency

Glybera® was approved by the EMA in 2012, making it the first gene therapy approved in Europe. It treats lipoprotein lipase deficiency, a rare inherited metabolic disorder in which the body cannot properly break down fat molecules called chylomicrons, leading to extremely high blood fat levels and recurrent, potentially fatal pancreatitis (inflammation of the pancreas).

Glybera® uses an AAV1 vector and is administered by intramuscular injection in multiple doses. Clinical studies demonstrated improvement in postprandial chylomicron metabolism (how the body handles fat after a meal), long-term expression of the LPL (lipoprotein lipase) gene, and the presence of active LPL protein. There was also a decreased trend in the incidence and severity of pancreatitis. It has also been investigated for familial hyperchylomicronemia.

The Current Clinical Trial Landscape

In addition to the 13 approved products, more than 200 active clinical trials are investigating in vivo viral vector-based gene therapies. These trials span all major viral vector types and disease categories.

The analysis in this review shows that AAV-based therapies are the most heavily investigated for monogenic diseases, reflecting AAV's advantages of long-term expression, low immunogenicity, and the ability to transduce both dividing and non-dividing cells. Ad-based therapies dominate the vaccine space, with recent COVID-19 approvals accelerating this field. HSV-based therapies remain focused on oncology, leveraging the virus's natural oncolytic properties. Lentivirus-based in vivo therapies are being explored for neurological, ophthalmological, and metabolic diseases.

The current pipeline suggests that the number of approved viral gene therapies will continue to grow. Many trials are studying approved products for new indications. For example, IMLYGIC® is being studied in more than 30 active trials for cancers beyond melanoma, and Gendicine® is being tested in combination with immune checkpoint inhibitors.

What These Approvals Mean for Patients

These approvals represent a fundamental shift in how certain diseases are treated. For patients with conditions that were previously considered incurable, gene therapy offers the possibility of a one-time treatment that provides long-term benefit.

• For cancer patients: Oncolytic viral therapies like IMLYGIC® provide a new treatment option for melanoma that cannot be surgically removed. Patients treated with IMLYGIC® had a response rate of 16.3% versus 2.1% with the control therapy, and overall survival improved from 18.9 months to 23.3 months. Gendicine® achieved a 90% total response rate in head and neck cancer, significantly higher than chemotherapy alone.
• For families affected by inherited diseases: Zolgensma® allows infants with SMA to reach developmental milestones like head control and sitting without support — abilities that would otherwise be lost to the disease. Luxturna® restores meaningful functional vision in children with RPE65-related blindness.
• For individuals at risk of infectious diseases: The Ebola vaccine Ervebo® showed 100% efficacy, and the COVID-19 vaccines demonstrated 65%–79% efficacy in their pivotal trials, providing protection against a virus that has killed millions worldwide.
• For patients with rare metabolic diseases: Glybera® demonstrated improvement in fat metabolism and a decreased trend in pancreatitis episodes for patients with lipoprotein lipase deficiency.

It is important for patients to understand that these are not conventional drugs that must be taken daily. They are one-time or limited-dose treatments designed to provide lasting benefit. However, they are also complex, often expensive, and require specialized medical centers for administration.

Challenges in Viral Vector Gene Therapy

Despite the remarkable progress, the clinical translation of in vivo viral vector-based gene therapies faces significant obstacles. The history of the field — including the 1999 patient death from immune responses and the 2000 development of leukemia in four patients — underscores the importance of safety.

The major challenges include:

• Immunogenicity: Viral vectors, particularly adenoviruses, can trigger strong immune responses. This limits the effectiveness of the therapy and, in severe cases, can be dangerous. It also means that patients who have pre-existing antibodies against a specific virus (from natural infection) may not respond well to a vector based on that virus. This is why some vaccine regimens, such as Zabdeno®/Mvabea® and Sputnik V, use different vectors for the first and second doses.
• Insertional mutagenesis: Retrovirus vectors integrate their DNA into the host genome at random locations, which can disrupt normal genes and cause cancer. This was the cause of leukemia in the four patients treated in 2000. Self-inactivating vectors reduce but do not eliminate this risk.
• Limited cargo capacity: AAV vectors can carry only about 4.7 kb of DNA, which limits their use for larger genes. HSV vectors can carry much more (over 100 kb in amplicon form) but are more complex to engineer.
• Transient expression: Ad vectors do not integrate into the host genome, so gene expression is temporary. This is acceptable for vaccines and some cancer therapies but not for lifelong correction of genetic diseases.
• Targeting specificity: Delivering the gene to the right cells or organs remains challenging. While different AAV serotypes have natural tropism for different tissues, achieving precise targeting often requires additional engineering.
• Durability of response: For vaccines, questions remain about how long protection lasts. For example, antibody responses to the Ebola vaccine declined from 90.0%–97.8% at 1 month to 83.2%–95.4% at 6 months.

The review notes that this is a rapidly evolving field, and strategies to address these challenges are actively being developed, including engineering viral capsids (the protein shells of viruses) to reduce immunogenicity and improve targeting, and using self-inactivating designs to reduce genotoxicity.

Recommendations for Patients and Families

For patients considering gene therapy or participating in clinical trials, the following considerations are important:

1. Understand the mechanism: Gene therapy is fundamentally different from conventional medication. It aims to correct the underlying genetic cause of a disease rather than manage symptoms. Ask your doctor to explain exactly how the proposed therapy works and what it can and cannot achieve.
2. Ask about the vector: Each viral vector has different characteristics. AAV-based therapies provide long-term expression with low immunogenicity but have limited cargo capacity. Ad-based therapies are well suited for vaccines and cancer but produce transient expression. Ask which vector is used in your proposed therapy and why it was chosen.
3. Discuss the evidence: Review the clinical trial data with your doctor. Ask about efficacy rates (for example, response rates, survival improvements, or milestone achievements), the duration of benefit, and the side effect profile. Ask what the numbers mean in absolute terms — for example, "how many of 100 patients benefit?" rather than just relative improvements.
4. Consider the logistics: Gene therapies often require specialized centers, specific administration routes (intravenous, intratumoral, subretinal, or intramuscular), and careful monitoring. Discuss travel, cost, and follow-up requirements.
5. Explore clinical trials: For conditions without approved gene therapies, more than 200 active clinical trials are investigating viral vector-based approaches. Clinicaltrials.gov is a reliable resource for finding trials. Ask your doctor whether you might be eligible and what participation involves.
6. Ask about long-term follow-up: Because gene therapy is relatively new, long-term safety data are still being collected. Ask about the expected duration of monitoring and what signs or symptoms should prompt you to contact your care team.
7. If considering vaccination: The viral vector-based COVID-19 and Ebola vaccines are well tolerated and provide meaningful protection. Understand the efficacy data in the context of your own health status and discuss any concerns with your healthcare provider.

Frequently Asked Questions

What is in vivo gene therapy and how is it different from other gene therapy?

In vivo gene therapy directly infuses or injects gene therapeutics into the patient's bloodstream or target organs. Unlike ex vivo therapy, it does not remove cells from the body, modify them in a laboratory, and return them. Engineered viruses are typically used as delivery vehicles for the corrective genetic material.

What viral vectors are used in approved gene therapies, and what are their main differences?

Approved products use adeno-associated virus (AAV), adenovirus, herpes simplex virus, retrovirus, or lentivirus. AAV enables long-term, low-immunogenicity expression and is used for inherited eye, neurological, and metabolic diseases. Adenovirus and herpes simplex virus are mainly used for cancer and vaccines. Retrovirus and lentivirus can integrate into the genome.

How do approved viral gene therapies for cancer work?

Some, like IMLYGIC and Oncorine, are oncolytic viruses that selectively replicate in and kill cancer cells. Others, like Gendicine, deliver a tumor suppressor gene (p53) into tumor cells to trigger cell death. Rexin-G delivers a cell-killing cyclin G1 gene. All are administered locally or intravenously depending on the product.

What were the results of the pivotal trial for IMLYGIC in melanoma?

In the late-stage trial leading to approval, IMLYGIC had a significantly higher response rate: 16.3% versus 2.1% with control therapy. Overall survival improved from 18.9 months to 23.3 months. It was approved for recurrent unresectable melanoma that cannot be removed by surgery, injected directly into lesions.

What do approved gene therapies for inherited blindness, spinal muscular atrophy, and metabolic disease offer?

Luxturna improved functional vision in children with RPE65-related inherited blindness. Zolgensma helped infants with spinal muscular atrophy reach developmental milestones like head control and sitting. Glybera improved fat metabolism and showed a decreased trend in pancreatitis episodes for lipoprotein lipase deficiency. All use AAV vectors and are one-time or limited-dose treatments.

What are the main risks or challenges with viral vector gene therapy?

Risks include strong immune responses to the vector, insertional mutagenesis if the DNA randomly integrates, limited cargo capacity, transient expression, and difficulty targeting the right cells. The article notes that a patient died in 1999 from immune responses and four developed leukemia in 2000 after retrovirus-based therapy, underscoring safety concerns.

If my condition has no approved gene therapy, are there clinical trials I could consider?

Yes. More than 200 active clinical trials are investigating in vivo viral vector-based gene therapies across all major vector types and disease categories. ClinicalTrials.gov is a reliable resource for finding trials. Ask your doctor whether you might be eligible and what participation involves, including long-term safety follow-up.

Should I get a second opinion before starting viral vector gene therapy for cancer or an inherited disease?

Gene therapy is a one-time or limited-dose treatment that aims to correct the underlying genetic cause of disease, not just manage symptoms. Approved products for cancer and inherited conditions have shown meaningful benefits, such as improved survival in melanoma and reaching motor milestones in spinal muscular atrophy, but therapies are complex, expensive, and carry risks like immune responses or insertional mutagenesis. A second opinion can help you review the clinical evidence, understand which viral vector was chosen and why, and explore whether the therapy is necessary or whether alternatives or clinical trials exist. Diagnostic Detectives Network provides independent expert second opinions.

Source Information

This patient-friendly article is based on the following peer-reviewed research publication:

Original Title: Viral vector-based gene therapies in the clinic

Authors: Zongmin Zhao, Aaron C. Anselmo, and Samir Mitragotri

Journal: Bioengineering & Translational Medicine, 2022;7:e10258

Publication Details: Received June 16, 2021; Revised August 4, 2021; Accepted August 11, 2021. Published by Wiley Periodicals LLC on behalf of the American Institute of Chemical Engineers. DOI: 10.1002/btm2.10258

Affiliations: University of Illinois at Chicago; University of North Carolina at Chapel Hill; Harvard University; Wyss Institute for Biologically Inspired Medicine.

Note: This patient-friendly article is based on peer-reviewed research. It is intended for educational purposes and does not constitute medical advice. Patients should consult their healthcare providers regarding any treatment decisions.