Researchers have engineered a virus that attacks cancer cells when injected into patients, a form of treatment that could kill tumors without harming healthy cells.

Scientists have long theorized that viruses could be used as a weapon against cancer but did not until recently have the ability to construct viruses that specifically target tumors. Now, researchers have for the first time created a virus that, once injected into a tumor, can replicate itself and spare non-cancerous cells.

We are very excited because this is the first time in medical history that a viral therapy has been shown to consistently and selectively replicate in cancer tissue after intravenous infusion in humans, said senior scientist at Ottawa Hospital Research Institution and lead researcher John Bell.

Researchers used the vaccinia virus, which was also employed to develop a smallpox vaccine, to create a new virus called JX-594. The trial administered JX-594 to 23 patients with advanced cancers, and seven of the eight patients receiving the highest dose saw the virus replicate itself within their tumors. Six of those patients also had their tumors stabilize or shrink.

While Bell acknowledged that the research is still in its early phases, he noted that the targeted virus treatment has one big advantage over chemotherapy: it would affect only cancer cells, avoiding the general destruction caused by radiation treatment.

With chemotherapy you get drastic side effects, said Dr. John Bell. Patients on this treatment only had 24-hour flu symptoms, and nothing after that.

The treatment could also help doctors to treat elusive cancer cells that quietly metastasize before they are found and treated.

Viruses have a great capacity for finding cells in certain parts of the body. They often tend to infect only certain types of cells, said William Phelps, director of preclinical and translational cancer research for the American Cancer Society . If we can manipulate that and take advantage of the natural capacity of the virus to spread throughout the body and to very selectively infect only certain types of cells, that could be very promising.

The experiment was conducted by researchers at the University of Ottawa and privately-held biotech company Jennerex Inc.