Gene Therapy

Gene therapy is a clinical approach that seeks to treat or prevent disease by introducing, removing, or modifying genetic material within a patient’s cells. Its success depends on the precise delivery of therapeutic nucleic acids into target cells while minimizing off-target effects and unintended biological responses. Effective gene delivery requires that genetic cargo cross the cell membrane, reach the appropriate intracellular compartment, and achieve controlled expression and stability to sustain the desired therapeutic effect. Among current delivery systems, viral vectors have become the predominant platforms due to their natural efficiency in entering cells and transferring genetic material. These engineered viruses can be designed for either transient or long-term transgene expression, depending on their underlying biology. Regulatory-approved gene therapy products primarily employ viral vectors derived from adeno-associated virus, adenovirus, lentivirus, retrovirus, and herpes simplex virus. These vectors and their role in gene therapy will be further discussed below. 

In vivo gene therapy involves the direct delivery of genetic material into a patient’s body, where the therapeutic transgene is taken up and expressed by the target cells. This method of administration enables the treatment of tissues that cannot be easily removed or cultured outside the body, such as the liver, muscle, eye, and central nervous system. The success of in vivo gene delivery depends on the ability to achieve efficient transduction, targeted tissue specificity, and durable gene expression while minimizing immune and off-target effects.

In vivo gene therapy products employ either systemic or localized delivery depending on their target tissue and therapeutic mechanism. Systemic, one-time intravenous infusions are used for AAV-based therapies such as Beqvez, Elevidys, Hemgenix, Roctavian, and Zolgensma, allowing widespread gene delivery via circulation. Localized administration is used for tissue-specific or oncolytic therapies, enabling more targeted gene expression while minimizing systemic exposure. These routes include subretinal injection (Luxturna), intravesical (Adstiladrin), intratumoral or intralesional (Gendicine, Oncorine, Imlygic), and topical or subcutaneous (Vyjuvek, Papzimeos).

Adeno-associated virus (AAV)-based gene therapy relies on the use of AAV and recombinant AAV (rAAV) vectors to deliver therapeutic transgenes into target cells. AAVs are small, nonpathogenic, single-stranded DNA viruses with a genome of approximately 4.8 kilobases. Recombinant AAVs are engineered to remove viral genes responsible for replication and integration, resulting in vectors that persist primarily as episomal forms rather than integrating into the host genome. This property allows stable, long-term transgene expression in non-dividing cells while minimizing the risk of insertional mutagenesis. AAV vectors are further valued for their low immunogenicity, versatility in tissue tropism, and capacity to deliver a wide range of therapeutic genes. These advantages have led to several successful clinical applications and regulatory approvals, including Beqvez (fidanacogene elaparvovec-dzkt) for hemophilia B, Elevidys (delandistrogene moxeparvovec-rokl) for Duchenne muscular dystrophy, and Luxturna (voretigene neparvovec-rzyl) for inherited retinal dystrophy.

The broader adoption of AAV vectors for gene therapies is limited by several scientific and engineering challenges. One constraint is the vector’s limited packaging capacity, as AAV efficiently accommodates transgene payloads only up to about 4.7 kilobases. Delivering larger genes requires splitting them into multiple vector genomes, which introduces complexity in ensuring accurate co-delivery and reassembly of the full-length transgene in target cells. Pre-existing immunity is another obstacle, as AAVs are naturally occurring human viruses and many individuals possess neutralizing antibodies that can inactivate the vector before it delivers its therapeutic cargo. This immune response also complicates repeat dosing, which is often necessary for sustained therapeutic benefit. To address these limitations, ongoing research focuses on AAV capsid engineering to create novel variants with enhanced transduction efficiency, reduced immunogenicity, and the ability to target a wider range of tissues, thereby expanding the clinical potential of AAV-mediated gene therapy.

Lentiviral (LV)-based gene therapy uses modified lentiviruses as integrating vectors capable of delivering up to about 9 kilobases of genetic material for stable, long-term expression of therapeutic transgenes. Unlike gamma-retroviral systems, LV vectors can transduce both dividing and non-dividing cells, enabling use in a broad range of tissues including neurons and hematopoietic stem cells. Their ability to express multiple genes from a single construct supports complex therapeutic designs, and specialized producer cell lines now allow scalable, high-titer vector production. Derived primarily from HIV-1, LV systems have been engineered across generations to enhance safety through removal of accessory genes, addition of self-inactivating elements, and use of split-packaging systems that prevent replication-competent virus formation.

Current and emerging research in lentiviral vectors focus on improving safety, integration control, and clinical versatility. Modern LV systems exhibit low immunogenicity, reduced cytotoxicity, and semi-random integration patterns that lessen the risk of insertional mutagenesis. Clinically approved applications are presently ex vivo, using lentiviral transduction of autologous hematopoietic stem cells or T cells for CAR-T and genetic disorder therapies. Future directions aim to expand in vivo use through integration-deficient vectors, site-specific integration technologies, and capsid or promoter engineering to enhance targeting and expression control. 

Herpes simplex virus (HSV)-based gene therapy employs engineered HSV vectors to deliver large genetic payloads for therapeutic or oncolytic purposes. HSV is an enveloped double-stranded DNA virus with a natural genome size of approximately 152 kilobases and a capacity to accommodate over 30 kilobases of foreign DNA, making it one of the most versatile platforms for gene delivery. Engineered HSV amplicons can package up to 150 kilobases of exogenous material, enabling delivery of multiple genes, regulatory sequences, or complex expression cassettes. Unlike integrating viral systems, HSV vectors form episomes within the host nucleus and remain extrachromosomal, minimizing risks of insertional mutagenesis while supporting durable transgene expression. While early HSV-based systems exhibited considerable cytopathogenicity, this has been mitigated through targeted deletions of non-essential viral genes and genome modifications that attenuate lytic activity, thereby improving safety and tolerability in clinical use.

Current and emerging HSV vector research focuses on expanding therapeutic precision, optimizing safety, and enhancing packaging efficiency. The development of helper virus-free production systems has improved vector purity and scalability for clinical manufacturing. Incorporation of regulatory elements such as microRNA target sequences has yielded conditionally replicating oncolytic HSVs capable of selective tumor cell killing, particularly in cancers such as non-small cell lung carcinoma. Clinically, HSV-based therapies have already achieved significant milestones with the approvals of Imlygic (talimogene laherparepvec) for melanoma and Vyjuvek (beremagene geperpavec-svdt) for dystrophic epidermolysis bullosa. Future directions include engineering vectors with improved tropism, reduced neurotoxicity, and the ability to deliver genome-editing machinery or combination immunotherapies.

The development of gene therapy products requires adherence to rigorous regulatory standards to ensure product quality, consistency, and patient safety. The U.S. FDA provides detailed guidances for drug developers outlining the expectations for characterization, manufacturing, preclinical testing, and clinical evaluation of gene therapy products. Each product’s testing strategy must be tailored to its unique features, particularly the nature of the vector and transgene. For viral vector–based products, such as AAV therapies, characterization may include the detection of non-vector DNA impurities within capsids using next gen sequencing, assessment of vector genome size and integrity, and evaluation of capsid protein modifications via mass spectrometry. These are critical for establishing product identity, purity, and potency, and their performance must be adequate even for initial Investigational New Drug (IND) submissions.

Accurate dose determination and assay qualification are central to regulatory compliance. Developers must validate the methods used to quantify active vector particles or transduced cells, such as vector genome titer by quantitative PCR (qPCR), transducing unit assays, plaque-forming unit (PFU) assays, or transduced cell quantification. In preclinical models, developers should evaluate the vector’s biodistribution, tissue tropism, and pharmacological activity of both the vector and transgene product. The permissiveness or susceptibility of animal species to the vector must also be assessed to ensure translational relevance of toxicology and efficacy data. Tumorigenicity testing, is important to address the risk of oncogenic transformation arising from vector integration or long-term transgene expression. Determining the need for such studies depends on the integration potential, transgene function, and persistence profile of the vector in vivo. Collectively, these regulatory considerations aim to balance innovation in gene therapy development with the highest standards of safety, reproducibility, and clinical accountability.