In today's era, where precision and innovation drive medical advancements, viruses have transitioned from being mere pathogens to allies in disease treatment. Viral therapy is emerging as a prominent research area for the treatment of cancer, genetic disorders, and immune-related illnesses. This article delves into the intricacies of viral therapy, exploring its categories, mechanisms, applications, and future directions, offering readers a comprehensive understanding of this groundbreaking medical technology.

Viral Therapy Definition: Understanding the Three Main Categories

Viral therapy, or virotherapy, represents a cutting-edge approach where viruses are transformed into therapeutic agents using biotechnology. By re-engineering viruses to possess disease-fighting capabilities, this innovative method has evolved over decades into three primary branches, each offering unique mechanisms and applications.

• Oncolytic Virus Therapy: Among the most direct applications of viral therapy, oncolytic virus therapy involves using naturally occurring or genetically modified viruses to selectively infect and destroy cancer cells, sparing healthy cells. These viruses are adept at targeting specific receptors overly expressed on cancer cells, such as the CD46 receptor on multiple myeloma cells. Once inside, the virus capitalizes on the rapid division of cancer cells to replicate abundantly, leading to the eventual rupture and death of these cells. Importantly, this process releases tumor antigens and danger signals, activating the patient's immune system against the cancer and establishing a lasting "vaccine effect."

The interactions between microglia/macrophages and oncolytic viruses (OVs) in gliomas. (Liu, Junfeng, et al. 2023)

• Viral Vectors for Gene Therapy: As the second main branch, viral vectors for gene therapy have achieved considerable clinical success. These vectors are modified viruses with pathogenic genes removed, retaining only their ability to efficiently deliver genetic material. Depending on the treatment strategy, they can facilitate four basic gene therapy modalities: gene replacement (substituting defective genes with functional ones), gene silencing (suppressing disease-causing gene expression), gene addition (introducing new genes to alter cellular functions), and gene editing (direct modification of genome sequences). Unlike oncolytic viruses, these vectors act as "molecular couriers," delivering therapeutic genes precisely to target cells.

Different methods to deliver therapeutic DNA and proteins to target cells. (Goswami, Reena, et al., 2019)

• Viral Immunotherapy: The third category, viral immunotherapy, skillfully blends viruses with immune therapy. Viruses serve as "alarm systems" for the immune system, mimicking natural infections to trigger an immune response against tumors or other diseases. Genetically altered viruses can carry genes for immune stimulators like GM-CSF, expressing these factors upon infecting tumor cells to draw immune cells into the tumor environment. This approach is particularly effective for tumors with immunosuppressive microenvironments, reawakening an immune system previously "paralyzed" by the tumor.

Employing OVs as adjuvants to synergize with multiple cancer immunotherapies. (Shi, Tao, et al., 2020)

Table 1: Three Main Types of Viral Therapy and Their Characteristics

These three branches of virotherapy, despite their differing mechanisms, harness the virus's natural cell penetration and gene delivery efficiency, transforming potentially harmful microbes into powerful treatment tools. As our understanding of viral biology deepens, these therapies continue to be optimized, offering new solutions for challenging diseases.

The Evolution of Viral Therapy

The history of virotherapy is a scientific odyssey marked by chance discoveries, setbacks, and breakthroughs. From initially observing the unexpected effects of viral infections on tumor growth to the approval of sophisticated gene therapy vectors and oncolytic virus products, the field has progressed through several pivotal phases.

Early Observations and Exploration (1940s-1950s)

In the mid-20th century, physicians noticed that some cancer patients experienced tumor shrinkage following viral infections. This intriguing phenomenon sparked interest in viruses as potential anticancer tools. Although early research was rudimentary, it laid the groundwork for the concept of oncolytic virus therapy.

The Emergence of Gene Therapy (1960s-1990s)

With advances in molecular biology, scientists began to envision viruses as vectors for delivering therapeutic genes. In the 1980s, retroviral vectors were successfully used in gene therapy experiments, and in 1990, the first gene therapy clinical trial successfully treated a child with ADA-SCID, marking the entry of virotherapy into clinical exploration.

Setbacks and Safety Improvements (1990s-2000s)

A major setback occurred in 1999 when a gene therapy trial using an adenoviral vector resulted in a patient's death. Additionally, retroviruses were found to potentially trigger leukemia. These events plunged the field into a period of low momentum, prompting scientists to enhance vector designs for better safety and control, such as developing self-inactivating (SIN) lentiviral vectors to reduce insertional mutagenesis risks.

Modern Resurgence and Precision (2010s-Present)

Post-2010, virotherapy has seen explosive growth:

• In 2015, the first oncolytic virus therapy, T-VEC, received approval for melanoma treatment.
• In 2019, AAV gene therapy Zolgensma hit the market, offering a potential one-time cure for spinal muscular atrophy.
• In the 2020s, viral vectors found extensive use in CAR-T cell therapies and vaccine development, including adenoviral COVID-19 vaccines.

Today, virotherapy is part of the precision medicine era, utilizing gene editing, targeted tissue delivery, and synthetic biology to achieve safer and more effective treatments. Companies like CD Genomics offer viral genome sequencing and AAV integration site analysis services, further driving optimization and clinical translation of viral vectors.

This journey from serendipitous discovery to modern precision medicine epitomizes the brilliance and perseverance of scientists, paving the way for tackling more intractable diseases in the future.

Viral Therapy Core Mechanisms of Action

The mechanisms underlying viral therapy are rooted in shared molecular and cellular processes, illustrating how viruses transition from pathogens to therapeutic tools. While different viral therapies have unique features, they all adhere to the following core principles:

Precise Recognition of Host Cells

The initiation of viral therapy is the specific recognition and binding of the virus particle to target cells. Viruses use surface proteins (like AAV capsid proteins or HSV glycoproteins) to identify specific receptors on target cells. Scientists can leverage natural viral tropism or genetically engineer surface proteins to enhance targeting accuracy towards diseased cells. For instance, engineered measles virus can specifically recognize the CD46 receptor abundantly expressed on multiple myeloma cells, ensuring precise targeting of the therapeutic tool to diseased areas.

Delivery and Expression of Genetic Material

Upon entering the cell, a virus's primary function is to deliver therapeutic genetic material. Modified viral vectors retain only necessary cis-acting elements (such as AAV ITRs), replacing viral coding genes with therapeutic gene expression cassettes. These exogenous genes can encode tumor-lytic factors, immunomodulatory proteins, or replacements for defective human genes. For example, T-VEC carries the GM-CSF gene to locally stimulate immune reactions in tumors, while AAV vectors used in spinal muscular atrophy treatment deliver the functional SMN1 gene.

Selective Replication and Cell Lysis (Oncolytic Viruses)

Oncolytic viruses selectively replicate within tumor cells and cause their lysis, driven by: overexpression of viral receptors on tumor cells; rapid proliferation providing an ideal replication environment; and loss of antiviral defenses. When viral replication reaches a critical point, tumor cells burst, releasing progeny viruses to infect nearby cells, amplifying the therapeutic effect. This process not only kills tumor cells but also releases antigens to initiate further immune responses.

Activation and Regulation of the Immune System

Viruses act as "danger signals" to activate the immune system through various mechanisms: tumor lysis releases antigens for uptake by antigen-presenting cells; viral components activate innate immunity via pattern recognition receptors; viral-encoded immunomodulators recruit effector immune cells. Clinically, oncolytic virus therapy has been observed to enhance immune cell infiltration in tumors, even turning "cold" tumor immune microenvironments "hot," and establishing long-lasting immune memory effects.

Summary of viral gene therapy modalities. (Bulcha, J.T., et al., 2021).

Table 2: Summarizing The Mechanisms of Action in Viral Therapy

These sophisticated mechanisms demonstrate how viral traits are harnessed for therapeutic advantages. With deeper understanding, scientists can rationally design the next generation of therapeutic products, tailored to the characteristics of various diseases, achieving comprehensive multi-tiered treatment effects.

Key Types of Viral Vectors

The effectiveness of viral therapy is closely tied to the type of viral vector used. These vectors, engineered from different viruses, offer unique advantages and applications, particularly in fields such as gene therapy and cancer immunotherapy.

Summary of viral vectors used in clinical trials. (Bulcha, J.T., et al., 2021).

1. Adeno-Associated Virus (AAV)

• Characteristics: Non-integrating, low immunogenicity, and provides sustained gene expression, making it suitable for long-term treatments.
• Limitations: Limited packaging capacity (<4.7kb) and potential to trigger immune responses.
• Applications: Rare genetic disorders, retinal diseases, central nervous system diseases.
• Representative Products: Zolgensma, Luxturna, Roctavian.

2. Adenovirus

• Characteristics: High transduction efficiency, large capacity (8–36kb), non-integrating.
• Limitations: High immunogenicity with transient expression, restricting repeat administration.
• Applications: Vaccines (e.g., COVID-19 vaccines), cancer immunotherapy.
• Representative Products: Oxford COVID-19 vaccine.

3. Lentivirus / Retrovirus

• Characteristics: Can integrate into the host genome for long-term expression, ideal for ex vivo therapies.
• Limitations: Integration poses risks of insertional mutagenesis and potential oncogenesis.
• Applications: CAR-T cell therapy, treatment of rare genetic diseases.
• Representative Products: Strimvelis, ZYNTEGLO.

4. Oncolytic Viruses

• Characteristics: Selectively infect and lyse tumor cells, activating the immune system.
• Types: Includes HSV (T-VEC), measles virus, VSV, adenovirus, among others.
• Applications: Solid tumor treatment, often combined with immune checkpoint inhibitors.

Comparison of Vector Characteristics

These viral vectors, each with distinct properties, are instrumental in advancing therapeutic options and addressing unmet medical needs through strategic application and design improvements.

Viral Therapy Clinical Success Stories

Viral therapy has transitioned from laboratory research to clinical applications, giving rise to a series of approved treatments that offer hope to patients previously deemed untreatable. These milestone products have validated the feasibility of viral therapy and set benchmarks for safety and efficacy in the field.

Breakthroughs in Cancer Treatment

• T-VEC: T-VEC is based on a modified HSV-1 virus, designed by removing pathogenic genes and incorporating the GM-CSF gene. Approved in 2015 for inoperable melanoma, it effectively reduces both primary and distant lesions, showcasing systemic immune activation capabilities.
• In bladder cancer treatment, the OH2 virus (modified HSV-2) administered via bladder infusion shows promising outcomes for high-risk patients unresponsive to BCG therapy, with a six-month recurrence-free survival rate of 66.7%, alongside a favourable safety profile.

Revolution in Genetic Disease Treatment

• Zolgensma (AAV9 vector): Treats spinal muscular atrophy through a single intravenous infusion delivering the SMN1 gene, significantly enhancing motor function in children, with some reaching normal developmental milestones.
• Luxturna (AAV2 vector): The first approved gene therapy for inherited retinal diseases, Luxturna addresses Leber congenital amaurosis caused by RPE65 mutations, significantly improving visual function in patients.

Innovative Treatments for Blood Disorders

• CAR-T Cell Therapy: Utilizes lentiviral vectors to modify T cells to express CAR receptors targeting tumors. The 2024-approved AUCATZYL® for B-cell acute lymphoblastic leukemia is the first CAR-T product not requiring special risk management protocols.

Expansion into Other Fields

• Haemophilia: AAV5 vectors deliver the clotting factor VIII gene.
• Metabolic Diseases: Lentivirus-modified hematopoietic stem cells treat metachromatic leukodystrophy.

These successful clinical applications underscore the transformative potential of viral therapies, advancing treatment across diverse medical fields and providing new solutions for challenging conditions.

Challenges and Future Directions in Viral Therapy

Challenges

While viral therapy has made remarkable strides, it still faces numerous challenges. In production and manufacturing, the large-scale production of viral vectors lacks standardized protocols. Current AAV production technologies have distinct limitations: transient transfection systems are flexible but difficult to scale up, baculovirus systems allow for scalability but produce post-translational modifications that differ from humans, and stable cell lines have lengthy development cycles. These bottlenecks contribute to high treatment costs, exemplified by Zolgensma's price of $2.1 million. Oncolytic virus production demands stringent control over the quality of replicative virus particles, where instability can directly impact therapeutic efficacy.

Safety and immunogenicity are prominent concerns as well. Pre-existing antibodies prevent treatment in 30-70% of patients, integration vector-related insertional mutagenesis risk, and off-target effects of oncolytic viruses are areas needing vigilance. Scientists are tackling these challenges by developing synthetic capsids and techniques like plasmapheresis, while improving vector designs to enhance safety.

Future Directions

Looking into the future, viral therapy is evolving towards precision. Technologies like CRISPR allow viral vectors not only to deliver genes but also to precisely correct mutations. New vector designs, such as dual-AAVs and hybrid viral vectors, overcome traditional capacity limitations, and capsid variants obtained through directed evolution significantly improve tissue targeting. Furthermore, the application domain is expanding from rare to common diseases, with continuous innovations in delivery methods.

With technological advancements, viral therapy promises significant future breakthroughs. Standardized and automated manufacturing processes will increase yield and reduce costs, transitioning this therapy from a custom solution to a mainstream treatment. The gene and cell therapy market is poised for sustained growth, with AAV gene therapy entering new developmental phases and oncolytic viruses gaining more attention as critical components of cancer immunotherapy. These advancements will ultimately unlock the tremendous potential of viral therapies to cure diseases, bringing hope to a wider patient population.