A patient with supposedly incurable cancer is now in complete remission thanks to a new experimental treatment she received at the Mayo Clinic: Killing cancer cells with massive doses of the measles virus.

The patient suffered from multiple myeloma, a cancer of the bone marrow. Last June, doctors at the famous Minnesota clinic injected her bloodstream with a form of measles that was genetically re-engineered to attack myeloma cells.

The measles therapy followed a decade of unsuccessful treatments from numerous courses of chemotherapy to two stem cell transplants. But the cancer returned time and time again — until now. A year after the measles injections, she's still cancer-free.

The idea of using viruses to defeat cancer — called oncolytic virotherapy — is not a new idea. But Dr. Stephen Russell, a professor of molecular medicine who treated the patient at the Mayo Clinic, says her case is the “first well-documented instance of a patient who has received an intravenously administered virus that has caused complete remission of disseminating cancer."

Russell calls the apparent success a milestone. “We've known for a long time that this is possible in mice, but we had not known that it’s possible in people," he says. "We now know it's possible and this should energize the field — but we have a lot of work to do.”

Oncolytic virotherapy works by exploiting the fact that cancer cells usually have weak ability to fight off infections.

“You can think of viruses as tiny biological weapons,” says Dr. Balveen Kaur, a professor at Ohio State and the vice-chairwoman of research at the school's Comprehensive Cancer Center.

“Viruses can infect normal cells, but it’s a self-limiting infection, so normal cells can easily overpower these viruses and get rid of the infection,” she explains. But in the weakened cancer cells, the virus can “replicate, destroy cancer cells [and] make more new virus, which can then go and kill more cancer cells around it.”

The doctors at Mayo used a strain of the measles virus that has been used in vaccinations since 1954 and “taught it to grow on human cancer cells ... That's how it became specific for cancer,” Russell explains.

He emphasizes that the treatment is different from a vaccine. “When you administer a vaccine, you give the minimum dose you can give in order to alert the immune system," he says. "We are using the virus as a weapon. We give it into a vein and we ask those viruses to seek and destroy the cancer cells. There are a large number of cancer cells in the body, so you need to give a massive dose of virus.”

Russell and his team have given this treatment to six patients, but only one of them had a complete response to the treatment. One had a partial response and the others no response at all. The distinction between a complete response and a cure, Russell says, is an important one.

“When we talk about complete responses in cancer therapy, we talk about getting the cancer to the point where we are unable to detect any cancer in the body. That doesn't mean there’s none there," he exlplains. "A great deal of the management of cancer is trying to prevent the cancer from coming back, because we suspect it will even though there are no detectable cancer cells in the body.”

One of the barriers to successful virotherapy treatment is the body’s own immune system. If an antibody to a virus is present in the patient’s blood stream, it will negate the benefit of giving the virus. Russell says the hope is to have a variety of viruses to use so that doctors can always find ones patients aren't already immune to.

Dr. Kaur is exploring another promising avenue of treatment that combines virotherapy with other cancer-fighting drugs. Cancer cells often develop a resistance to chemotherapy drugs, but Kaur performed a study that found that drug-resistance in cells makes them “extra sensitive to viral infection and replication.”

Now she and other researchers are experimenting with a “two-pronged approach,” in which chemotherapy destroys some of the cancer cells while cells developing drug resistance are targeted with virotherapy.

For Dr. Russell and his team at Mayo, the next step is a Phase II clinical study which will “test the same high dose of virus in a larger number of patients with advanced multiple myeloma who have no treatment options. We will select the patients who do not have the anti-measles antibody in their blood, and we will see whether this can really be replicated. “

“Hopefully,” he says, “by combining the virus with drugs or by increasing the dose further still, we will be able to get those complete cures which we would love to see.”

This story first aired on PRI's Science Friday.