While pox viruses call to mind images of pus-filled blisters and painful sores, a few clever genetic tweaks turn these viruses from deadly villains into cancer-fighting heroes.
Best known as the live virus present in the smallpox vaccine, vaccinia virus is surprisingly good at killing cancer cells. With a natural affinity for tumors, a fast replication time, and a good safety profile, pox viruses have the potential to treat deadly and treatment-resistant cancers, including pancreatic and brain cancers.
“We're presenting a virus that basically saved the world from the scourge of smallpox,” said Nicholas Lemoine, a cancer researcher at VacV Biotherapeutics and Queen Mary University of London who is developing a vaccinia virus-based cancer treatment. “What actually came from vaccination has shown us that this type of platform is something we should never forget about.”
Cancer fighting viruses work by infecting tumor cells and doing what viruses do best: making more of themselves. After co-opting the tumor’s cellular machinery to replicate their genetic material, the viruses burst out of the cell, leaving an explosion of dead cancer cell debris in their wake. Patrolling antigen presenting cells of the immune system pick up these bits of dead cancer cells and use them to prime other immune cells like T cells to recognize and destroy cancerous cells.
“The cancer cells themselves are basically giving up their arms and presenting them to the invading army of the adaptive immune response,” Lemoine said. By stimulating the host’s immune system to recognize tumors, these viruses make it possible for T cells to find and kill tumors that may have spread to other parts of the body.
Scientists recognized the potential anticancer effects of viruses when they discovered the first oncolytic virus in 1992. When they injected a genetically modified herpes simplex virus into a brain tumor in a rat, the tumors shrank significantly (1). Since then, researchers have tested a variety of different cancer fighting viruses in preclinical and clinical trials, and regulators have approved four for use in humans so far. Regulators in Latvia approved an unmodified picornavirus to treat melanoma in 2004, and the National Medical Products Administration approved adenovirus H101 to treat head and neck cancer in China in 2005 (2). Other regulatory bodies, however, have not approved these treatments in other places yet. In the United States, Europe, and Canada, regulators approved a modified herpes simplex virus called T-VEC (brand name Imlygic) to treat certain melanomas in 2015. In 2021, Japanese regulators gave limited approval to another modified herpes simplex virus named Teserpaturev/G47Δ (brand name Delytact) for brain cancer treatment.
Although approved, the main challenge with these virus-based cancer treatments is that they’re not always effective. Recent research on vaccinia virus in combination with other drugs suggests that the pox virus may be a safe and more effective solution.
Due to its use in the smallpox eradication campaign in the 1960s and 70s, the world already considers vaccinia virus quite safe. But the virus also has multiple inherent features that make it particularly adept at targeting cancer.
For one, vaccinia virus has a natural preference for infecting cancer cells over normal ones, which scientists noticed when fluorescently labeled virus accumulated in a mouse brain tumor but not in healthy cells (3). Unlike many other viruses, vaccinia virus doesn’t need to bind to a specific cellular receptor to enter a host cell; it infects them via endocytosis (4). Vaccinia virus also thrives in low oxygen conditions often associated with solid tumors (5). Once inside the cell, vaccinia virus replicates in the cytoplasm, avoiding any interaction with host cell DNA in the nucleus. This behavior led many scientists to consider the pox virus safer than therapies based on herpes virus and adenovirus, which replicate in the nucleus.
The cancer cells themselves are basically giving up their arms and presenting them to the invading army of the adaptive immune response.
- Nicholas Lemoine, VacV Biotherapeutics and Queen Mary University of London
Researchers wanted to improve vaccinia virus’s tumor specificity even more. While the virus encodes a number of different genes to help it replicate inside host cells, “people discovered that those genes are kind of redundant for the virus to infect and replicate in cancer cells,” said Zong Sheng Guo, a cancer researcher at Roswell Park Comprehensive Cancer Center.
One of these viral genes codes for the enzyme thymidine kinase, which catalyzes a critical step in the synthesis of thymidine triphosphate. Because cancerous cells rapidly divide, they have a large pool of thymidine nucleotides for the virus to use. To make vaccinia virus more selective for cancer cells, scientists deleted its thymidine kinase gene, ensuring that the virus can only replicate inside and lyse cancer cells but not healthy cells, which divide less frequently and therefore don’t maintain a pool of thymidine.
These advantages propelled vaccinia virus-based therapies into multiple clinical trials in recent years, but like their other oncolytic viral counterparts, their potency has been low. One reason for this may be that alone, vaccinia virus lysis of cancer cells may not stimulate the host immune system as well as it needs to. Researchers wondered if pairing vaccinia viral therapy with immune checkpoint inhibitors or other immune modulating treatments might amplify its power.
“I have been interested in combination [therapies] with small molecules targeting different key signal pathways,” said Guo. Using a vaccinia virus that he and his colleagues developed, Guo and his team administered the virus with a small molecule activator of ferroptosis, an iron-dependent cell death pathway, to mouse models of hepatocellular carcinoma and colon cancer (6,7). The combination treatment shrank the tumors more than either treatment alone.
“It's a great result. We like it. We just need to know more [about] how they work together, and in the future, we can improve our strategy to enhance the efficacy even further,” said Guo.
The best studied vaccinia viral treatment is JX-594 (brand name Pexa-Vec), a vaccinia virus expressing the immune-activating gene granulocyte-macrophage colony-stimulating factor (GM-CSF) and an inactivated thymidine kinase gene.
Researchers at the National Institutes of Health (NIH) led by cancer researcher Tim Greten recently presented data from a phase 1/2 study using Pexa-Vec in combination with one of two different immune checkpoint inhibitors in patients with refractory colorectal cancer (8). Only some types of colorectal cancer respond to immune checkpoint inhibitors, but Greten hoped that combining the treatments with the oncolytic vaccinia virus would improve outcomes for patients with advanced colorectal cancer.
“I was hoping for a stronger response,” Greten said. “We had a patient who showed a significant response and a long-lasting response, and you ask, why is it one and not the others?”
Despite the lackluster results, Greten and his team still believe in the oncolytic virus strategy to help treat these difficult cancers. “They work,” he said. “There's a lot of data suggesting that this type of treatment actually does make sense.” He and his team plan to keep testing different combinations of the virus and immune modulating drugs to find the most effective treatment for cancer patients.
Although not as far along as Pexa-Vec, another vaccinia virus strategy developed by Lemoine and his colleague Yaohe Wang at VacV Biotherapeutics recently demonstrated positive results in mouse and hamster cancer models.
Wang’s team recently discovered a way to increase the amount of vaccinia virus that infects tumors. Vaccinia virus can form extracellular enveloped virions (EEVs) where the virus hides inside an envelope made up of the tumor cell’s surface membrane, disguising the virus from the immune system. The researchers noticed that when they delivered the vaccinia virus treatment intravenously, host macrophages cleared most of the virus before it arrived at the tumor (9). To take up the vaccinia virus, macrophages depended on an enzyme called PI3 kinase (PI3K) delta. When the team intravenously administered both their vaccinia virus and a clinically licensed PI3K inhibitor, they saw increased vaccinia virus at the tumor, and tumor growth slowed in mouse models of colorectal or breast cancer, increasing their survival.
“The PI3 kinase delta inhibitor is essentially rolling the pitch in order to suppress macrophages, which would otherwise engulf the viruses that you then deliver,” said Lemoine.
Wang and Lemoine recently tested their vaccinia virus expressing the cytokine interleukin-21 in mice with glioblastoma (10). When they injected the mice intravenously with this engineered virus and also gave mice the immune checkpoint inhibitor α-PD1, the treatment slowed tumor growth and, in some cases, eliminated tumors altogether. The mice also survived longer.
The team also slowed pancreatic tumor growth with their engineered vaccinia virus combined with α-PD1 in mice and hamsters (11). In particular, the researchers found that when used together, the vaccinia virus sensitized tumors that were previously resistant to α-PD1 treatment by priming the immune system.
“When we found positive results in tumor systems that had basically failed to respond to anything else, we redoubled our efforts to bring this to clinic,” said Lemoine. The team plans to begin clinical trials sometime in mid-2024.
“If we can use this platform to save a huge number of people from bad outcomes with cancer using a virus that basically saved the world from smallpox, I think that's a good story to tell,” said Lemoine.
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- Lauer, U.M. and Beil, J. Oncolytic viruses: challenges and considerations in an evolving clinical landscape. Future Oncol 18, 2713-2732 (2022).
- Yu, Y. et al. Visualization of tumors and metastases in live animals with bacteria and vaccinia virus encoding light-emitting proteins. Nat Biotechnol 22, 313-320 (2004).
- Mercer, J. and Helenius, A. Vaccinia virus uses macropinocytosis and apoptotic mimicry to enter host cells. Science 320, 531-535 (2008).
- Hiley, C. et al. Lister strain vaccinia virus, a potential therapeutic vector targeting hypoxic tumours. Gene Ther 17, 281-287 (2010).
- Kowalsky, S.J. et al. Superagonist IL-15-Armed Oncolytic Virus Elicits Potent Antitumor Immunity and Therapy That Are Enhanced with PD-1 Blockade. Mol Ther 26, 2476-2486 (2018).
- Liu, W. et al. Ferroptosis Inducer Improves the Efficacy of Oncolytic Virus-Mediated Cancer Immunotherapy. Biomedicines 10, 1425 (2022).
- Monge, C. et al. A phase I/II study of JX-594 oncolytic virus in combination with immune checkpoint inhibition in refractory colorectal cancer. European Journal of Cancer 138, S57-S58 (2020).
- Ferguson, M.S. et al. Transient Inhibition of PI3Kδ Enhances the Therapeutic Effect of Intravenous Delivery of Oncolytic Vaccinia Virus. Mol Ther 28, 1263-1275 (2020)
- Sun, Y. et al. An effective therapeutic regime for treatment of glioma using oncolytic vaccinia virus expressing IL-21 in combination with immune checkpoint inhibition. Mol Ther Oncolytics 26, 105-119 (2022).
- Marelli, G. et al. A systemically deliverable Vaccinia virus with increased capacity for intertumoral and intratumoral spread effectively treats pancreatic cancer. J Immunother Cancer 9, e001624 (2021).