A genetically-engineered virus has prolonged the lives of terminally ill liver cancer patients in a small clinical trial, according to the findings published in Nature Medicine. The results will need confirming in larger studies, but Cancer Research U.K. welcomed the advance as "an exciting step forward."

The partially disabled cowpox virus, known as Pexa-Vec or JX-594, was given to 30 patients with advanced liver cancer during the study. Sixteen patients given a high dose of the vaccine survived for an average of 14.1 months, compared with 6.7 months for the 14 who received the low dose.

“For the first time in medical history we have shown that a genetically engineered virus can improve survival of cancer patients,” said study co-author Dr. David Kirn, a researcher at Jennerex Biotherapeutics in San Francisco.

Professor Alan Melcher, a Cancer Research U.K. expert at the University of Leeds who is also working on modified viruses to treat cancer, was upbeat about the research but cautioned that the work was still a several years from the clinic.

He said, "It helps demonstrate the cancer-fighting potential of viruses, which have relatively few side effects in comparison to traditional cancer treatments such as chemotherapy or radiotherapy. If this treatment proves effective in further, larger trials, then it could be available to patients within five years."

JX-594 has been genetically engineered from the vaccinia virus, which has been used as a vaccine for decades, helping to eradicate smallpox. It is designed to multiply in and subsequently destroy cancer cells, while at the same time mobilizing a patient's own immune defense against the cancer.

Patients with a type of advanced liver cancer called hepatocellular carcinoma had both a reduction in the size of their tumor and a decreased tumor blood flow when treated with either a low or high dose. For most patients, side effects were relatively mild at both at high and low doses, with all participants suffering from a day or two of flu-like symptoms, while one patient had severe nausea and vomiting.

A larger trial is needed to confirm the results, and a follow-up with approximately 120 patients is already under way in the U.S. The virus is also being tested in other cancer types.