The highly localized nature of the new treatment may help. "By expressing the natural peptide in only the area of the nervous system that's responsible for transmitting the pain sensation," says Fink, "we can block the pain without having those side effects."

The technique has proved safe and effective in rodents, and Fink anticipates that these results will translate well to human patients. For the clinical trial, the enkephalin gene has been paired with a molecular switch that will deactivate it after a few weeks, in case any adverse reactions occur. Fink's group has also developed switches that can continue to churn out enkephalin for months or even years.

While the University of Michigan study, sponsored by the Swedish biotech company Diamyd Medical, is the first to enter clinical trials, several other groups have been developing related gene therapies for treating chronic pain. In January, researchers at Mount Sinai School of Medicine announced that by injecting a virus carrying an opioid gene directly into spinal fluid, they successfully treated chronic pain in rats. Compared with that approach, the HSV-based therapy is far less invasive: the former is akin to a spinal tap, the latter to a flu shot.

HSV's benefits may also pose a limitation, however. Many cancer patients experience pain throughout the body, not just at the site of the tumor. In those cases, localized relief is not enough. "This type of approach is not suited for pain all over the body," says Fink.

Nonetheless, Fink believes that gene therapy will open up an entirely new way to approach chronic pain. "We understand a lot about how pain is perceived and how it works in the nervous system," he says, "but we haven't made tremendous progress on finding new ways to treat pain."

Khan agrees that gene therapy can offer fresh treatment possibilities where old tactics have failed. "I see so many people who experience these chronic unremitting pains," he says. "With the current tools that we have, our hands are really tied."