The virus that causes cold sores can be used to shrink — or even obliterate — melanoma tumors, according to a new study that relied on two dozen Minnesota patients with the potentially deadly skin cancer.

Patients receiving injections of the modified virus were more likely to remain cancer-free after treatment, and those with certain stage 3 and 4 melanomas were less likely to die in subsequent years, said Dr. Thomas Amatruda, a cancer specialist at Minnesota Oncology and a study author.

“You see (tumors) shrinking week after week,” he said. “It is one of the most satisfying things I do in my practice.”

The St. Paul-based cancer practice was one of the largest among 86 international sites to test the experimental therapy, named T-Vec, which will be reviewed by the U.S. Food and Drug Administration this fall to determine whether it can be prescribed normally to patients.

Approval would make T-Vec the first in a wave of immunology treatments that use modified viruses to stimulate the immune system to target and eliminate tumors. Several are hot topics at this weekend’s conference of the American Society of Clinical Oncology, including a modified polio virus to treat a brain cancer known as glioblastoma, and a cowpox virus to target head and neck cancers.

Melanoma has been an increasing target for research because of its frequency in an era of indoor tanning and other exposures to ultraviolet light. Its prevalence in Minnesota doubled between 1990 and 2010. More than 1,500 cases are newly diagnosed each year, according to recent data from the Minnesota Department of Health.

Triggers immune system

T-Vec is based on the herpes simplex virus type 1, which is commonly responsible for oral cold sores. When injected directly into tumors, or remnants of tumors after surgery, the modified virus weakens the cancer cells while also turning them into identifiable targets for the immune system.

Researchers hope the viral treatment not only eliminates tumors, but creates the kind of one-and-done immunity that people get after exposure to chickenpox, which prevents new tumors from emerging.

“In cancer, the problem is never just what you see. It’s what you don’t see,” Amatruda said.

In the study of 436 melanoma patients, researchers sought to substantially shrink or eliminate the tumors for at least six months. This happened with 16 percent of patients who received the viral therapy, compared with 2 percent of patients who received a comparative therapy that also tried to boost the immune system. But the results were much better when focusing on patients who had reached stage 3 transitions, in which their cancers had spread to nearby lymph nodes, bones and muscles — but not to other organs. Results were detailed this week in the Journal of Clinical Oncology.

‘At the end of my rope’

Karilyn Johnson of Watertown, Minn., said she felt out of options in 2009 after undergoing surgery and taking a harsh drug known as interferon that failed to eliminate the cluster of melanoma tumors on her head.

Trying the experimental viral therapy seemed her best shot, Johnson said, especially considering that her insurance wasn’t going to cover another treatment option and the recession had taken a toll on her family’s income.

“I kind of felt at the end of my rope,” she said.

Johnson, 68, said the series of experimental injections into each tumor was painful. At one point, Amatruda asked if the pricks felt like mosquito bites. “Yeah,” he recalled her saying as she spread her arms wide. “Mosquitoes with stingers this big!”

Tumors disappeared

But within weeks, the tumors disappeared. Five years later, they haven’t returned. Johnson has returned to work in an assisted-living facility and to horse-riding in an arena and trails near her home.

Now Johnson takes lumps on her head more seriously. Before her first diagnosis, she thought they were merely a reaction to shampoo. “I’m always checking,” she said.

Determining who should receive T-Vec, if it gets federal approval, will be challenging. While cost estimates aren’t yet available, the price is expected to be high, given the decade of development that its manufacturer, Amgen, needed to bring the therapy this far. The cost-benefit question has emerged with many new cancer treatments, especially those that offer slight benefits or extend life by weeks and come with harsh side effects.

Amatruda, who has received previous financial compensation from Amgen for consulting, said he expects follow-up studies to verify that T-Vec is worthwhile for patients with late-stage melanomas — particularly those who see tumors emerge after initial ones are surgically removed.

“These people (in the study) already were kind of the hard cases,” he said, “where they had come back and we thought they were going to keep coming back.”