A rabbit virus slowed the growth of multiple myeloma in mice, and eradicated the cancer in a fourth of the animals, a study reported.
But researchers cautioned that the road to using the virus in humans would be long and uncertain.
An advantage that a virus would have over chemotherapy is that cancer would be unlikely to develop resistance to it. That’s because viruses have evolved over thousands of years to thwart the tricks that cells use to fight it.
The study, “Systemic therapy with oncolytic myxoma virus cures established residual multiple myeloma in mice,” was published in the journal Molecular Therapy — Oncolytics.
“What I thought was really interesting here was that we could actually get rid of disease and it didn’t appear to ever come back,” Eric C. Bartee, PhD, an assistant professor of microbiology and immunology at Medical University of South Carolina (MUSC), said in a press release.
The researchers have been working with the rabbit microbe, called Myxoma virus, for some time. In an earlier report, they showed that the virus, which doesn’t infect humans, killed myeloma cells in a lab dish.
That discovery prompted them to see if the virus could eliminate the few myeloma cells that remained when stem cells were harvested for a transplant. They learned that it could.
The finding in the earlier study was exciting because most people who relapse in myeloma do so because the cancer was not completely eliminated from their body.
To see if the virus could eliminate residual myeloma cells in a living organism as well as a lab, the researchers injected myeloma mice with the virus. Within 24 hours, the number of myeloma cells plummeted between 70% and 90%.
This led to delayed disease progression in 66 percent of the mice. Even more thrilling, the cancer was wiped out in 25 percent of the mice, and did not come back during an 80-day observation period.
Experiments showed that the killing of the cancer cells did not occur in bone marrow.
Since the myxoma virus is unable to spread once it enters myeloma cells, researchers believe an immune response toward the virus was what killed the cells. Their suspicion was supported by an increase in the number of immune-system T-cells after the virus was introduced.
Bartee cautioned that much work would need to be done before the treatment could be tried in humans. To start with, large quantities of clinical-grade virus would need to be produced, which would be a challenge.
In addition, the response rate to the virus therapy in humans would have to be much better than in the mice.
“I think the major next question is ‘How do you get that response rate from 25% to 50% to 80% to 100%?'” Bartee said. And “how do you define the patients in which it works?”
Since the immune system helps kill myeloma cells, researchers believe it could be possible to combine the virus with immunotherapy to increase response rates.
“I think what our findings, and oncolytics in general, really highlight is that some of these non-traditional therapies can really offer the benefit of complete disease eradication,” Bartee said. “You’re not just moving the curve to the right a little bit; you’re bending the survival curve up. And you’re really fundamentally changing how you can look at cancer treatment.”