6 Min Read
Last Updated: 02/02/2024
Fact Checked

Written by Karen Selby, RN | Medically Reviewed By Dr. Andrea Wolf | Edited By Walter Pacheco

What is Virotherapy?

Virotherapy is a cancer treatment that uses a virus to find and destroy cancer cells. It’s a targeted treatment that can deliver therapy without harming healthy cells.

Types of virotherapy include oncolytic virotherapy and viral immunotherapy. Another type is viral vectors, which is also called viral gene therapy.

How different types of virotherapy work:

  • Oncolytic virotherapy uses viruses to infect and destroy cancer cells without affecting normal cells.
  • Viral immunotherapy uses a virus to deliver an immune-stimulating substance called an antigen to the immune system.
  • Viral vectors are laboratory-modified viruses designed to alter cancer cell genes to treat the disease.

Viral vectors also can deliver tumor antigens into the body. Antigens are substances found on cancer cells but not on normal cells. This stimulates an antitumor immune response.

This is similar to how viral immunotherapy works. Sometimes the terms “viral vector therapy” and “viral immunotherapy” are used interchangeably.

Virotherapy is not a cure for mesothelioma. Researchers hope combining it with other therapies will help cancer patients live longer.

Origins of Virotherapy

More than a century ago, a surgeon named William Coley developed virotherapy. He noticed that cancer patients with an infection after surgery sometimes lived longer than those without.

Dr. Coley wondered if the infection triggered the patients’ immune system to attack the cancer. He developed experimental treatments called Coley’s toxins. Some consider these to be one of the first immunotherapy treatments.

Coley’s toxins first used live bacteria injected into tumors. Later, he used heat-killed microbes. The results were unpredictable. Some patients experienced severe infections and died after Coley’s treatments. Over time, the approach fell from favor.

Fortunately, Coley’s early work was not wholly forgotten. It laid the foundation for developing today’s immunotherapy and virotherapy cancer treatments.

How Virotherapy Works

Oncolytic Viruses

Oncolytic viruses infect cancer cells. Some viruses naturally infect cancer cells. Other viruses can be modified in the lab to target specific cancer cell types.

The viruses make their way into cancer cells and reproduce rapidly. The rapid viral reproduction ruptures the membranes of cancer cells and destroys them.

The destroyed cancer cells release antigens. These are substances more easily recognized as foreign by the body. This stimulates the immune system to attack remaining tumors, too.

Oncolytic viruses seem to destroy tumors in two ways. Firstly, by directly rupturing cancer cell membranes. Secondly, they indirectly stimulate the immune system to recognize and attack the cancer.

Researchers are also developing methods to combine gene therapy and virotherapy using oncolytic viruses. One example includes modifying oncolytic viruses to deliver p53 genes. These genes help the immune system fight cancer.

Antitumor Mechanisms of an Oncolytic Virus. The illustration depicts an oncolytic virus infecting a normal cell but unable to replicate, leaving the normal cell unharmed. A second example shows an oncolytic virus infecting a tumor cell, replicating and causing lysis which ruptures the tumor cell and promotes antitumor immune response.

Viral Immunotherapy

Viral immunotherapy uses a virus to deliver an immune-stimulating substance called an antigen into the body. The antigen helps the immune system recognize and attack cancer cells.

A 2014 phase I clinical trial report published in the medical journal Oncoimmunology describes this type of virotherapy. Researchers evaluated the case of a 68-year-old man with asbestos-related malignant pleural mesothelioma (MPM).

The clinical trial used a type of virus called an adenovirus. The treatment is called ONCOS-102.

When a response is better than expected, the doctors may publish a report detailing these results. In the ONCOS-102 clinical trial, the man survived 18 months from treatment beginning and more than 33 months from diagnosis.

The study scientists reported the response as “remarkable,” given the median survival of patients with MPM varies from 4 to 12 months from diagnosis.

Phase I studies test the safety, side effects, best dose and timing of a new treatment. The dose may be increased slowly to find the highest amount that does not cause harmful side effects.

Although phase I clinical trials are not designed to treat or cure patients, the ONCOS-12 clinical trial seemed to treat one participant more effectively than conventional therapy.

Viral Vectors

Viral vectors are created in the lab. Researchers start with a typical virus and alter it to create one that cannot cause disease.

This type of virotherapy is considered a form of gene therapy because the modified viruses alter genes in targeted cells. Targets can include cancer cells or other malfunctioning cells contributing to genetic diseases.

Some viral vectors target malfunctioning mesothelioma genes. One example of this approach uses a virus to disrupt how mesothelioma cells create the proteins that allow uncontrolled cell growth — a hallmark of all cancers.

In another approach, scientists have created a virus targeting mesothelioma genes that make too much of one growth factor. The virus only infects the cells with excessive amounts of the growth-promoting substance.

Viral vectors can kill cancer cells directly or make them more sensitive to conventional treatments such as chemotherapy and radiation therapy.

Mesothelioma Virotherapy Clinical Trials

Promising viruses for mesothelioma virotherapy include modified versions of vaccinia, measles, adenovirus, herpes simplex and Newcastle disease virus.

These approaches have been studied in clinical trials, though none is approved for widespread clinical use yet. These treatments are only available to mesothelioma patients participating in clinical trials.

A 2021 clinical research study studied the CpHV-1 virus as a potential new mesothelioma virotherapy approach. The researchers found that CpHV-1 strongly synergized with cisplatin, a standard chemotherapy medication, and this agent combination did not affect healthy mesothelial cells.

Clinicaltrials.gov currently lists numerous clinical trials using virotherapy approaches. Searching for “mesothelioma and virotherapy” will return results on these trials.

These studies are funded by pharmaceutical companies, cancer centers and government research. For example, the National Cancer Institute has funded some of these studies, including a virotherapy mesothelioma clinical trial.

Additionally, specific research centers are conducting virotherapy research for mesothelioma. For example, the National Institutes of Health and Vaccine Immunotherapy Center conduct mesothelioma research on immunotherapy and virotherapy.

Combining Virotherapy and Conventional Cancer Treatment

There are few studies on combinations of virotherapy and chemotherapy. Preclinical (animal) research suggests this approach may be very effective for treating mesothelioma.

One example of preclinical combination therapy research studied mesothelioma virotherapy plus pemetrexed (Alimta) and cisplatin or carboplatin. Pemetrexed and platin chemotherapy drugs are the standards of care.

Medical experts accept this standard of care as appropriate treatment for particular cancer. It is widely used by medical oncology professionals and typically is the first treatment offered when a person is diagnosed with mesothelioma.

This study found that chemotherapy alone did not decrease mesothelioma tumor growth. However, the virotherapy alone slowed tumor growth, and the combination of virotherapy plus chemotherapy delayed tumor growth even more.

The safety and immune-activating properties of this virotherapy in humans are already known. Chemotherapy is already the standard of care and is widely used to treat mesothelioma.

These factors provide a strong rationale for clinical trials testing virotherapy with first-line chemotherapy in patients with malignant mesothelioma. These trials likely will be coming soon.

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